A ringback number is a telephone number for a telephone line that automatically calls the line that the call was placed from, after the caller has hung up. The typical use of this facility is by telephone company technicians for testing a new installation or for trouble-shooting..
Operation
Typically, a technician tests the terminal equipment, such as a telephone set, by calling a ringback telephone number or a vertical service code. When the exchange answers the call, the switch may play a signal tone, at which time the technician flashes the line once or multiple times. A higher pitched tone then indicates ringback is engaged, and the technician hangs up. The exchange then places a call to the originating line, causing that telephone to ring. When answered, the exchange plays a verification tone. The verification tone is issued so that telephone subscribers cannot easily use the ringback system as an intercom between multiple stations in a residence on the same line by taking the stations off-hook. Ringback numbers are typically not listed or communicated to subscribers.
Coin telephones
and other criminal activity can utilize payphones to conduct crimes anonymously, so many payphones are not equipped with a ringer at all, or a quiet "chirper" solid state speaker and designated "No Incoming Calls". All payphones in the United States must be assigned a telephone number in order to make calls, some can ring or chirp if that number is called. Many customer-owned coin-operated telephones answer the phone at first ring with a built in modem which can be accessed by technicians to report conditions and program function parameters, one programmable function is the number of rings until the modem answers, another is whether to impose an additional charge for incoming calls or even accept no incoming calls at all. Where a payphone does not have any number listed on the unit, the number can be discovered from that phone by calling an automatic number announcement circuit service or a telephone with caller ID. TelCo owned payphone are generally on dedicated payphone lines which respond to special tones and Custom Local Area Signaling Services codes such as *5055 for ringback.
Examples
Some ringback numbers are local or regional in scope, while others are larger in scope. Every telephone company determines its own ringback numbers for each individual central office. Under the North American Numbering Plan, most North Americanarea codes reserve telephone numbers beginning with 958 and 959 for internal testing. Some companies also reserve 999 for test exchanges. Numbers within these test exchange block ranges are used for various types of local and long-distance testing; generally, this block includes a ringback number and a loop around. Ringback numbers may appear in the 958 exchange, but there is no requirement that they reside there. Some large telephone companies have toll-free numbers set up for ringback. In most cases, these numbers remain undisclosed to prevent abuse. Some companies change their ringback number every month to maintain secrecy. Some carriers have been known to disable all payphone calls to 958 or 959 test lines. 4101 was formerly valid on some mechanical switching systems to allow a call to the other party on a two-party line. Like 4104, it was once a standard number in many areas but has disappeared as this equipment has been decommissioned. In the 1970s and early 1980s, 1199011 was a number that when called would result in another dial tone. Afterwards, if the phone was hung up and then quickly picked up again in less than approximately 1/2 second, a steady tone would then be heard. Hanging up the phone after this would result in a ring back. When the ring back was answered, the ring back process could be repeated by hanging up and lifting up the phone quickly again, and then hanging up. Most other numbers listed for ringback are specific to one exchange or one telco; available lists tend to be outdated and unreliable. Many former test numbers have been reclaimed for use as standard landline or mobile exchange prefixes, with the test codes moved to the 958 exchange.
