The exchange then places a reverting call to the originating line, causing the telephone to ring.
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.
Every telephone company determines its own ringback numbers for each individual central office.
Some carriers (such as Bell Canada) 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 (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.