SS7
From aa-asterisk.org.uk wiki
SS7, or "Common Channel Signalling System No. 7" is an ITU-T standard for carriers to talk to each over over traditional telephone networks. SS7 does not carry voice data but the signalling protocols to indicate ringing, congestion, call progress, etc.
In the VoIP world it is very rarely used except for inter-operating with traditional carriers such as BT telephone exchanges. Asterisk does have an implementation of SS7 available, but it is not approved by the major carriers and is therefore unlikely to be used in production.
