ITU-T H.450.4 PDF

This Recommendation specifies the Call Hold supplementary service (SS-HOLD), which is applicable to various basic services supported by ITU-T H.323 Multimedia Endpoints.

SS-HOLD enables the Served (Holding) User A to put User B (with whom User A has an active call) into a hold condition (Held User) and subsequently to retrieve that User again.

During this hold condition, User B may be provided with music and/or video on hold. The Served (Holding) User A may perform other actions while User B is being held, e.g., consulting with another User C.

The call between User A and User B has to be in the active state before SS-HOLD may be invoked. This Recommendation makes use of the "Generic functional protocol for the support of supplementary services in ITU-T H.323 systems" as defined in Recommendation ITU-T H.450.1.