Fact Sheet 19:
Caller ID and My Privacy

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Copyright © 1996-2016
Privacy Rights Clearinghouse
Posted January 1996
Revised October 2014
  1. How does Caller ID work?
  2. How does blocking work?
  3. Are there exceptions to blocking?
  4. How can I choose the best blocking option?
  5. How do I know if the person or business I am calling subscribes to Caller ID?
  6. Can numbers be captured even if the phone is not answered?
  7. My phone doesn't display any phone numbers. How can I see the phone numbers of people calling me?
  8. What if I choose Per-Line Blocking, but some people won't answer the phone unless they can see my number?
  9. What if I switch blocking options? Does it cost anything to change my mind?
  10. If I move and order new phone service, must I re-order Caller ID blocking?
  11. My home has more than one phone number. Do I have to choose a blocking option for each one?
  12. Can I block the phone number when I make calls from my cellular phone?
  13. Can phone numbers be blocked from payphones?
  14. Will blocking work with modems? What about fax numbers?
  15. Does blocking work at my workplace? What about hotels and hospitals?
  16. How can I find out what type of blocking has been selected on the phone I am calling from?
  17. What can marketers do with my telephone number?
  18. What Is Anonymous Call Rejection?
  19. Is Caller ID offered everywhere?
  20. What happens when I call someone long distance in another state?
  21. Is Caller ID useful in stopping harassing phone calls
  22. Are there other ways to stop harassing phone calls?
  23. If I have complaints about Caller ID service, who can I contact?


Caller Identification, or Caller ID, acts like an electronic peephole, allowing a person receiving a phone call to see who is calling before answering the phone. The caller’s telephone number and/or name is displayed either on your phone (if your phone has this feature) or on a display unit that you must buy separately. The number and/or name appears after the first ring.

All cell phones have this feature, but it’s optional on landline phones for an extra fee if it’s not among the features included in your rate plan.  Most of the information in this fact sheet pertains to landline phones. Features on cell phones will vary depending upon your provider. You should contact your carrier for information specific to your phone service.

Since its introduction, Caller ID has become popular as a way to screen for unwanted calls. It is often used by consumers wishing to avoid telemarketers and those with personal safety concerns.

These devices raise privacy issues for both the caller and the person being called. For one thing, Caller ID has become easier to subvert. The ability to send out a false, misleading number to the person called — known as “spoofing”— has serious implications for victims of stalking, harassment, and identity theft.

Furthermore, there’s the issue of whether you even want your telephone number being captured by the place you are calling. If someone does not pick up the phone, the Caller ID device will retain the number — even if it is unlisted — for future viewing . In addition, the time and date of the call are also recorded and displayed.

Caller ID means you lose control over the privacy of your phone number -- unless you take advantage of the free phone number blocking options that are available to you and which are explained below. It’s important to note that these features will not work in all situations, such as when calling toll-free numbers or for emergency assistance when using the 911 system.  In these cases, your telephone number will continue to be available to the person you are calling.

1. How does Caller ID work?

Your local phone company or cell phone carrier sends your “Calling Party Number” (CPN) with every call, much like a return address on an envelope. Transmitted along with your CPN is a privacy flag that tells the telephone switch at the receiving end whether to share your number with the recipient. If you have blocking on your line, the phone company you are dialing into knows your number but will not share it with the person you are calling.

If you have blocking and you call someone who subscribes to Caller ID, he or she will see the message “Private,” “P,” or “Anonymous” instead of your phone number. (See exceptions below.)

This arrangement relies on uniform software in the phone equipment at both ends of the call. In the United States this is backed by Federal Communications Commission (FCC) rules that dictate how phone carriers handle CPNs, Caller ID, and blocking. So most subscribers have come to take Caller ID for granted.

But the system has never been wholly secure. Businesses have been able to pay for the privilege of misrepresenting their phone numbers or, on the other side of equation, getting a toll-free number that can access an incoming caller’s number even if it is blocked.

Adding to the problem is that Voice over Internet Protocol (VoIP) networks — which allow calls to be routed over the Internet — is outside FCC regulation. Ordinary citizens using VoIP can fairly easily disguise where they are calling from, and they can subvert Caller ID blocking to learn the phone numbers of those who call them.  This is known as “spoofing.” To learn more about VoIP, read the FCC’s guide at www.fcc.gov/cgb/consumerfacts/voip.html.

On December 22, 2010, President Obama signed into law the Truth in Caller ID Act (S. 30, Public Law No: 111-331). The Act makes it unlawful, with some exceptions, for any person to cause any Caller ID system "to knowingly transmit misleading or inaccurate caller identification information with the intent to defraud, cause harm, or wrongfully obtain anything of value." The law applies to Caller ID used in connection with both telecommunications services and IP-enabled services (VoIP).

On June 23, 2011, the FCC adopted rules implementing the Truth in Caller ID Act. You can read more about implementation of the Act at http://www.fcc.gov/guides/caller-id-and-spoofing and at FCC Adopts Rules

2. How does blocking work?

FCC rules require local phone companies to make available free, simple, and uniform per-line blocking and unblocking processes.  These rules give you the choice of delivering or blocking your phone number for any interstate (between states) calls you make. (The FCC does not regulate intrastate calls.)

Some Caller ID services also transmit the name of the caller. If you request that your phone number be concealed, FCC rules require that a carrier also conceal your name.

The two blocking options are:

  • Per-Call Blocking. (Also known in some states as Selective Blocking.) To block your phone number or name from appearing on a recipient’s Caller ID unit on a single phone call, dial *67 before dialing the phone number. Your number ordinarily will not be sent to the other party. (See exceptions below.) But you must redial *67 each time you place another call.
  • Per-Line Blocking. (Also known in some states as Complete Blocking.) Some states allow customers to select per-line blocking so that their phone number will be blocked for every call they make on a specific line unless they use the per-line unblocking option. If you want the number transmitted to the called party, dial *82 before you dial the number you are calling. You must redial *82 each time you want to unblock. 

3.  Are there exceptions to blocking?

Several exceptions exist to the FCC blocking rules:

800 number/toll-free calls. You are not able to prevent the display of your phone number when you call 800, 844, 855, 866, 877, 888, and 900 numbers. The called party, which pays for the call, may be able to identify your phone number using Automatic Number Identification (ANI) technology. FCC rules, however, limit parties that own toll-free numbers from distributing the numbers identified through ANI.

Emergency services. Calls to emergency lines — like 911 — are exempted from federal Caller ID rules. When you call emergency numbers, your phone number is transmitted even if you have Per-Line Blocking. In fact, the display device used by emergency services usually shows not only your phone number, but also your address so that police, fire and ambulance services can locate you in case you’re unable to provide the address. 

Telemarketers. Under FCC rules, telemarketers are required to transmit Caller ID information and are prohibited from blocking such information.

4. How can I choose the best blocking option?

Your local phone company will assign you Per-Call Blocking unless you specify Per-Line blocking. So it’s important to make your choice known.

Choosing Per-Line Blocking. The higher the risk you face if your phone number is revealed, the more likely you will want to choose Per-Line Blocking, called Complete Blocking in some states including California. Your phone number will automatically be blocked for every call you make when you select Per-Line Blocking, though you may unblock individual calls by pressing *82 before dialing the number.

You may want to Per-Line Blocking if:

  • You have an unlisted phone number. In other words, you’ve already decided that telephone privacy is so important that you are willing to pay a monthly fee to keep your number private. You do not want your number being captured by people and businesses you call.

  • You are a victim of stalking, domestic violence, or another form of harassment and must block your number at all times.

  • You are a member of a profession in which the privacy of your home phone is important. You may at times need to make work-related calls from home and want to ensure that the persons you call are not able to reach you there. Some examples are health care professionals, especially mental health care providers; school teachers; law enforcement officers; probation officers who may call parolees from home; judges and other court officials; social workers; entertainers; IRS and other government employees.

  • You operate a domestic violence shelter or a safe home and must safeguard the phone number and location of the residents.

  • You want to report crimes to the police department's Crime Tips Line but wish to remain anonymous.

  • You have occasion to call a "help" hotline and want to remain anonymous. Such hotlines include those for suicide prevention, AIDS information, immigration assistance, and mental health issues. 

Here are some other situations where you might choose Per-Line Blocking:

  • You do a lot of price shopping from home and do not want your phone number collected for marketing purposes by the businesses you call.
  • You have visitors to your home who might use your phone, such as babysitters, your children's friends, relatives, and other guests. You don't want to risk revealing your number when such phone calls are made.
  • You are helping a friend or relative who is vulnerable to telemarketing scams like get-rich schemes, sweepstakes, and prize offers. To help them limit who is able to capture their phone number, you suggest they select Per-Line Blocking.
  • In general, you are very conscious of your privacy and want to take every precaution to safeguard it.

Choosing Per-Call Blocking. If you experience few of the risks listed above, you may wish to choose Per-Call Blocking for your home phone, called Selective Blocking in California and some other states. Your phone number will be sent to the parties you are calling unless you enter the code *67 before you dial. You will hear a "stutter" dial tone after entering *67

Situations in which Per-Call Blocking may make sense include:

  • Most of the people you call from home are friends and family. Those who subscribe to Caller ID do not like to pick up the phone unless they see the phone number of the caller.

  • You live alone, have few visitors, and are very careful about your use of Per-Call Blocking. You’re not likely to forget to enter the Selective Blocking code *67 when you need to shield your number.

  • You do not really know how you feel about phone number privacy, so you choose Per-Call Blocking on a trial basis to see how it works for you. You might change to Per-Line Blocking in the future if you find that you prefer a higher degree of privacy.

Remember, Per-Call Blocking puts you at risk for the occasional slip-up -- by yourself, guests, the babysitter, or children. A single call where you or a guest forget to enter the code (*67) can leave you open to unwanted marketing or, worse, a stalker or harasser.  When you visit others and use their phone, always ask which blocking option they use.

Make your choice known by calling your local phone company's toll-free blocking selection number. You will receive a confirmation from the phone company regarding the blocking option you selected. Remember, unless you choose Per-Line Blocking, which is the strongest option for people concerned about their privacy, you will automatically be given Per-Call Blocking.

5.  How do I know if the person or business I am calling subscribes to Caller ID?

There is no way to know if they have Caller ID. If you are concerned about your phone number being captured, you must use one of the blocking options listed above.

6.  Can numbers be captured even if the phone is not answered?

Yes, in most cases. This can allow, for example, a business to know who called after hours. But the incoming phone number is not captured in every instance. For example, if the line is busy when someone tries to call, the Caller ID display device will not capture the number. And it may not capture the number if the called party has Call Waiting.

7.  My phone doesn't display any phone numbers. How can I see the phone numbers of people calling me?

You can subscribe to the Caller ID service by paying a one-time start-up fee and a monthly charge to the local phone company. (Call your phone company's business office for cost information.) You must also buy a Caller ID device or a specially equipped telephone that displays incoming telephone numbers.

8.   What if I choose Per-Line Blocking, but some people won't answer the phone unless they can see my number?

You can selectively unblock your number and allow it to be transmitted. Enter *82 each time you want your phone number displayed.

9.  What if I switch blocking options? Does it cost anything to change my mind?

Usually, you get one change free and then a fee will be charged.

10. If I move and order new phone service, must I re-order Caller ID blocking?

To play it safe, tell the phone company which blocking option you want when you move.

11. My home has more than one phone number. Do I have to choose a blocking option for each one?

Yes. If you want Per-Line Blocking on each line, you will need to order it for each phone number in your home. If you do not specify which blocking option you want, you will be given Per-Call Blocking. Remember, blocking is free no matter how many phones you have.

12.  Can I block the phone number when I make calls from my cellular phone?

In most cases, no. But call your cellular provider to find out. Ask if it uses “Signal Switching 7,” or "SS7," the telephone switching technology that enables callers to block their phone numbers. 

13.  Can phone numbers be blocked from payphones?

All payphones are supposed to allow Per-Call Blocking. Blocking is only necessary when you put coins into the payphone. If the call is operator-assisted or involves a calling or credit card, the payphone's number is not transmitted to the recipient's phone; therefore, blocking is not needed.

14.  Will blocking work with modems? What about fax numbers?

If you do not want your modem's phone number to be transmitted, be sure to choose one of the blocking options. Fax numbers are a different story. Federal regulations require fax machines and fax cards in computers to display the fax number on the document that is sent. (47 CFR 68.318) So even if you block your fax number, it will still be printed on the recipient's document.

15.  Does blocking work at my workplace? What about hotels and hospitals?

If your workplace uses Centrex (AT&T) or Centranet (Verizon), you’re able to block your phone number. But blocking is not likely to work if your office uses a PBX system or if you are on a switchboard. The same is true for hotels and hospitals with PBX systems. If it is important to block your number from the office, a hotel or hospital -- for example if you are being stalked -- be sure to find out if blocking will work. Listen for the "stutter" dial tone after dialing *67, a sign that blocking works. You might want to make test calls to friends who have Caller ID.

16.  How can I find out what type of blocking has been selected on the phone I am calling from?

In California, when using AT&T, call (800) 386-0000. For Verizon, call (800) 483-8707. When you call this blocking-verification number from the phone you want to use, a recorded message will tell you which blocking method is in use for that phone line. This number does not work in other states. So if you travel outside of California, ask which blocking feature is available at the phones you use.

17. What can marketers do with my telephone number?

If you call companies to gather information about products, prices, or hours of service, your telephone number could be captured unless you block the number. Your phone number may have several uses:

  • The company could simply use your number to make sales calls to you.

  • The company might match your phone number against a data base containing names and addresses, and then mail you advertisements.

  • Your number could be sold to companies that compile lists for marketing purposes. The number would be matched against large data bases containing names, addresses, and consumer profile information (such as household income and ages). The enhanced mailing list information would be sold to direct marketers.

18.  What Is Anonymous Call Rejection?

Some phone companies provide Anonymous Call Rejection. This programs the phone to reject calls from anyone who blocks their phone number -- sometimes called "Block the Blocker." Callers hear an announcement that the subscriber does not accept blocked calls.

Individuals can purchase devices in stores that do the same thing as Anonymous Call Rejection. Instead of allowing the phone to ring, the caller hears a recorded message stating that they must call again without blocking the number.

19.  Is Caller ID offered everywhere?

Caller ID is available in all 50 states but may not be available in all areas of every state. If you are not sure if your area has Caller ID and other Custom Calling Services, ask your phone company's business office. Always read your bill inserts for information on these services. No matter where you live, if you want to be sure that your phone number is blocked, enter *67, the Per-Call Blocking code, before dialing the number.

When you travel, keep in mind that Per-Line Blocking may not be available everywhere. If you need to block your phone number while you are on the road, play it safe by entering *67 before you dial the number. This blocking code works in all 50 states.

20.  What happens when I call someone long distance in another state?

Caller ID crosses state lines, and so do the blocking codes. When you make long distance calls, your phone number is sent to the party you are calling, the same as if you're calling someone nearby. Unless you use Per-Line or Per-Call Blocking, your phone number will appear on Caller ID display devices in other states. In a few areas Caller ID technology is not yet installed. People who get calls from places where Caller ID is not yet available will see the message "Out of Area" on their display devices.

21.  Is Caller ID useful in stopping harassing phone calls?

Most harassers will block their phone numbers or will call from payphones. However, Caller ID can be used by people receiving harassing calls to decide whether to answer calls. They can choose not to pick up calls marked "private" or numbers they do not recognize.

22.  Are there other ways to stop harassing phone calls?

Yes. Other services are available, sometimes for an extra fee, from the phone company to help you deal with harassing callers. Products may also be available in stores for blocking harassing calls.

Call Trace.  The phone company's Call Trace service allows you to identify the phone number of the harassing caller, regardless of whether the caller blocks the number. When you activate the Call Trace code (*57) immediately following the harassing call, the caller's phone number is recorded by the phone company. The phone number is not given to you, however. You will be asked by the phone company to sign an authorization form before the harasser's phone number is turned over to law enforcement for further investigation.

To order the Call Trace service, you must pay a start-up fee. Some phone companies then charge you each time you use Call Trace; others require a monthly charge for an unlimited number of traces.

Call Trap.  When you report repeated obscene or harassing calls to the phone company's Annoyance Call Bureau, it can decide to put a Trap on the line at no cost to you. You will be asked to keep a log of the dates and exact times that you get harassing calls. The Trap will be kept on your line for a specific period of time, usually two weeks. When you give your log to the phone company, it will check the number against its records to determine the phone number(s) of the harasser and then give this information to law enforcement.

Choosing between Trap and Trace.  Instead of choosing Call Trap, the Annoyance Call Bureau might decide to provide you Call Trace at no charge for a two- to three-week period. However, in areas where Call Trace is not yet available, a Trap is the only way to track down the numbers of harassing callers.

Call Screen (sometimes called Call Block or Selective Call Rejection). This service allows you to block calls from up to 12 numbers, which you identify. You can also block the number which just called you, even though you may not know the actual number. Your phone will not ring when you are called from one of these numbers. Instead, the caller will get the message, "The party you are calling is not accepting this call." There is both a start-up fee and monthly charge for this service.
Be aware that if you use Call Screen to reject calls from a harasser, that person is likely to go to another phone or a telephone booth to circumvent the Call Screen.

Selective Call Acceptance. You can program your phone line to accept up to 12 phone numbers, those callers who you most want to talk to. All other callers, including the harasser, hear a message that you are not accepting calls now. You can turn this service on or off at any time.

Call Return.  This feature, in return for a start-up fee and monthly charge, lets you automatically redial the last call you received. Call Return is of limited use for harassing calls, however. When you enter the Call Return code *69 to dial the most recent call, you are not able to learn the number of the caller. (In some states, a voice message gives that number to you when you enter *69, but not in California.) If the returned call is a toll call, the last four digits of the number do not appear on your phone bill. But you will have to pay the toll charge. Another limitation is that Call Return only works in the local service area.

Special devices. Special call-screening devices known as "inbound call blockers" can be purchased that allow you to reject certain calls. Only those callers who enter a special numeric code onto their touchtone phones are able to ring through to your number. Victims of harassing calls have reported these devices to be very effective. However, you must be sure to give the code to everyone you want to talk to. You could miss important calls from unexpected sources, including emergency services. These and other types of call screening devices can be purchased at stores that sell consumer electronics devices.

Answering machine/service. Don't forget that the most effective call screening device for harassers may be the answering machine, an answering service, or voice mail.

For additional tips on dealing with harassing calls:

23.  If I have complaints about Caller ID service, who can I contact?

Complaints about Caller ID in can be made to your state’s public utilities commission.  In California, that agency is:

For a directory of the public utilities commissions in each state:

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