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.

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