SS7 is the ubiquitous communications protocol that is used within and between telephony networks of the world for system management, call setup and tear-down, and other network functions. Although there are slight variations in the fine detail of SS7 implementations used around the world, all telephone companies use the same set of protocols known generically as SS7.
Because the signaling is on a separate network to voice traffic, SS7 allows the use of features such as automatic callback when a called number is busy. Messages about the states of the voice lines are sent and received on the signaling network so voice lines are not tied up until both parties are known to be available to speak.
SS7 is used for many other functions within a telephone network. Essentially, when the computer in one telephone exchange or switch needs to talk to computers elsewhere in the network (either to other switches or to standalone machines such as those used for billing and advanced services), SS7 messages are always used.