MGCP is an internal protocol used within a distributed system that appears to the outside world as a single VoIP gateway.
This system is composed of a Call Agent, and a set of gateways, including at least one "media gateway" that performs the conversion of media signals between circuits and packets, and at least one "signaling gateway" when connected to an SS7 controlled network.
The "approved" IETF protocol for the same purpose, although not as widely implemented as MGCP, is Megaco, defined in RFC 3015.
This article was originally based on material from FOLDOC, used with permission. Update as needed.