Q.931 (also called Q931) is a signaling protocol for Integrated Services Digital Network (ISDN) communications that is used in voice over IP (VoIP). The Q.931 protocol is involved in the setup and termination of connections.
Q.931 is used to transmit and receive call signaling messages according to the H.225 protocol for digital telephone services. The messages in Q.931 include setup (a signal indicating the establishment of a connection), call-proceeding (a signal indicating that the call is being processed by the destination terminal), ring-alert (a signal that tells the calling party that the destination set is ringing), connect (a signal sent back to the source indicating that the intended destination phone set has received the call), and release/complete (a signal sent by either the source or the destination indicating that the call is to be terminated).