In today's world, there are a number of options for controlling who can reach an individual via telephone, as well as how and when. The process of controlling these factors falls under a broad category known as call privacy. Call privacy consists of a number of functions and services to stop or ignore calls that the phone’s owner does not want to receive. The availability of these calling features varies on different phones and with different telephone service providers.
One of the most well-known privacy features on phones is caller ID. This function, also known as calling line identification or calling number identification, was first introduced to the public by telephone company BellSouth in December 1988. Caller ID enables the person receiving a telephone call to see the number, and sometimes the name, of the individual who is calling. This enables the recipient of the call to pick up, ignore or send the call to an answering machine or service at their leisure.
There are a number of methods for blocking a telephone number so that it does not display on a caller ID screen. Call blocking is similar in some ways to caller ID. This means that the individual receiving the call is unable to tell who or what number placed the call they are receiving. Certain companies have a series of digits one can call, known as a vertical service code, to identify the call after it is completed. Other companies, particularly cell phone companies, do not offer vertical service codes as a function.
The term call block is also used to refer to a type of call privacy that enables the owner of a telephone to block all or some numbers. A feuding spouse may choose to block his partner's number, for example. A person whose call is being blocked hears a message that states the caller does not wish to receive calls at that time. This type of call blocking is usually enacted or reversed by calling the telephone company directly.
One common use for call privacy functions is to disable or ignore unwanted or unsolicited calls from telemarketing agencies. Within the United States, a national “Do Not Call” registry has been enacted wherein a phone owner can opt to not receive telephone marketing from a number of agencies. This does not stop all unsolicited calls, and the phone owner must be sure to read the privacy policy of each company they give their number to.