Ringback number
A ringback number is a telephone number used by telephone company linemen to test a new telephone line installation and that the assigned number is routed properly.
Typically a lineman tests the terminal equipment (telephone set) ringer by calling a ringback telephone number or a vertical service code, and then hanging up and waiting for the telephone to ring. When answering, the switch may play a low frequency signal tone, at which point the technician flashes the line (hangs up briefly) one or more times. A higher pitched tone then indicates ringback is engaged, and the technician hangs up. The phone will then ring, answering only with a verification tone when picked up. The verification tone is added so that telephone subscribers cannot easily use the ringback system as an intercom, to call other extensions of their own phone line by simply initiating a ringback test and waiting for the phone to stop ringing and converse when someone else on their own line has answered the extension.
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 American area 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 (to test the ringer when installing telephone sets) and a loop around (which connects a call to another inbound call to the same or another test number). 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 (such as Bell Canada) have been known to disable all payphone calls to 958 or 959 test lines.
Coin telephones
Narcotics trafficking 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 (COCOT) 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 (ANAC) 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.
Ringback numbers
4101 was formerly valid on some mechanical switching systems to allow a call to the other party on a two-party line. Like 4104 (repair, long replaced by 611 in most cities), it was once a standard number in many areas but has disappeared as this equipment (and the party line service itself) has been decommissioned.
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 (such as 320 and 999 in Bell Canada territory) have been reclaimed for use as standard landline or mobile exchange prefixes, with the test codes moved (usually) to the 958 exchange.