Which protocol allows Cisco Unified Communications Manager to take control of a specific port on a gateway?

SIP is a peer-to-peer signaling protocol where both endpoints participate in call setup, so CUCM does not directly control gateway ports.

H.323 is also a peer-to-peer VoIP protocol that negotiates calls between endpoints rather than allowing centralized port control.

Q.931 is the ISDN layer 3 call control protocol used over PRI circuits, not a protocol for a call manager to control gateway ports.

MGCP (Media Gateway Control Protocol) follows a master/slave architecture where the call agent (CUCM) maintains full control over specific physical and virtual ports on a gateway. Unlike peer-to-peer protocols, MGCP endpoints are dumb and rely entirely on CUCM for call signaling and port management, which is exactly what 'taking control of a specific port' describes.