Caller ID is a telephone service feature that allows the recipient of a call to see the phone number and, in some cases, the name of the person calling before answering. This information is displayed on the phone's screen, helping users decide whether to pick up the call or not. The technology works by sending the caller's information over the phone line during the ringing phase of the call. Caller ID can be useful for screening calls, avoiding unwanted telemarketing, and identifying important contacts. It is widely available on both landline and mobile phones.