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. 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.