The Mobile Services Switching Centre or MSC is a sophisticated telephone exchange which provides circuit-switched calling, mobility management, and GSM services to the mobile phones roaming within the area that it serves. This means voice, data and fax services, as well as SMS and call divert.

In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent directly digitally encoded to the MSC. Only at the MSC is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound encoded digitally as PCM signal in a 64-kbit/s timeslot, known as a DS0 in North America).

There are various different names for MSCs in different contexts which reflects their complex role in the network, all of these terms though could refer to the same MSC, but doing different things at different times.

A Gateway MSC is the MSC that determines, by location, which MSC to use when a subscriber is being called. It also interfaces with the Public Switched Telephone Network. All mobile to mobile calls and PSTN to mobile calls are routed through a GMSC. The term is only valid in the context of one call since any MSC may provide both the gateway function and the Visited MSC function, however, some manufacturers design dedicated high capacity MSCs which do not have any BSCs connected to them. These MSCs will then be the Gateway MSC for many of the calls they handle.

The ‘Visited’ MSC is the MSC where a customer is currently located. The VLR associated with this MSC will have the subscriber's data in it.

The Anchor MSC is the MSC from which a handover has been initiated. The Target MSC is the MSC toward which a Handover should take place.