In telephony, a service switching point (SSP) is the telephone exchange that initially responds, when a telephone caller dials a number, by sending a query to a central database called a service control point (SCP) so that the call can be handled.
7 (SS7) protocols which are responsible for the call setup, management, and termination with other service switching points.
With the introduction of the Intelligent Network architecture, service functionality (e.g. UK translation of 0800 non-geographic telephone numbers) is being removed from the actual telephone exchange and devolved out into other computer nodes.
This causes a trigger within the SSP that causes an SCP (Service Control Point) to be queried using SS7 protocols (INAP, TCAP).
The SCP responds with a geographic number, e.g. 0121 XXX XXXX, and the call is actually routed to a phone.