Ringback number

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Ringback number is a number used by many phone companies to test whether or not a new telephone line is working correctly and the phone number is routing properly.

Typically it works by calling a "ringback number" and then hanging up. Often the ringback number plays a low frequency tone into the line to indicate that it's working, at which point the technician "flashes" the line (hangs up very briefly) to indicate they want a ringback. This makes the ringback play a higher tone to indicate it is working, and then the technician hangs up. The phone will then ring, answering only with a tone if picked up.

The 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 (so as to know when the other person has answered).