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Ringback number is a number used by many telephone company linemen/technicians to test whether or not a new telephone line installation is working correctly and the phone number is routing properly. Typically a lineman tests the terminal equipment (telephone set) ringer by calling a "ringback telephone number" or ringback vertical service code and then hanging up and waiting for the phone to ring. Often upon connection the ringback number plays a low frequency signal tone into the line to indicate that it's working and ready, at which point the technician "flashes" the line (hangs up very briefly) one or more times to select a ringback. A higher pitched signal tone then indicates ringback is engaged, and the technician hangs up. The phone will then ring, answering only with a verification tone if picked up. The verification tone is added so that telephone subscribers will not be able to 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 has answered the extension. Some ringback numbers are very local or regional in scope, while others are state-/province- or area-code-wide: there appears to be no consistent national system for them. Every telephone company, whether large or small, determines its own ringback numbers for each individual central office, which tends to perpetuate the current situation of a mess of overlapping and/or spotty areas of coverage. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ringback number」の詳細全文を読む スポンサード リンク
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