SIP Call Hold

SIP Call Hold feature allows a User to generate a SUS message to put a call on hold or unplug a terminal from a socket and uses a RES message once the terminal is re-connected to a new socket and the T2 (User Initiated) or T6 (Network Initiated) timer has not expired. While the call is suspended, no audio media is passed end-to-end.

SUS Message (Suspend) - A SUS message is generated and sent in either direction to inform the called or calling party is temporarily disconnected. The Suspend (SUS) message indicates a temporary cessation of communication without releasing the call. SUS can only be accepted in an answered state (ICC answered/OGC answered).

RES Message (Resume) - A Resume (RES) message indicates a request to recommence communication. If the RES is not sent within timer T6 or T2 (T2 applies to ITU only), the controlling exchange will initiate a release procedure.

RFC 3398 Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation Protocol (SIP) Mapping: section 9 

Call Flows

Below are the call flows the IMG 2020 supports when utilizing the SIP Call Hold feature.

Suspend then Resume from SS7 Side

After a call is in the answered state, a SUS message is received from the remote SS7 side. The IMG 2020 sends a re-INVITE message to the remote SIP side with the Connection IP address set to 0.0.0.0 in order to request the gateway to place the call on hold. The call is now on hold and no audio is passed. Before the T2/T6 Timer can expire a RES message is sent with the IP information to reconnect to and the call resumes.

If the RES message is not received before the Timer expires, the IMG 2020 will release the call

Open

Suspend then Resume from SIP Side

After call is in an answered state, the IMG 2020 receives a Re-INVITE with the connection address equal to 0.0.0.0. The IMG 2020 then sends a CPG message with the notification indicator set to ‘remote hold’ to put Remote SS7 side on hold. When call is to resume, the remote SIP side sends a Re-INVITE with a valid IP address. The IMG 2020 sends a CPG message with the notification indicator set to ‘remote hold released’ to the remote SS7 ANSI side and the call is relased.

Open

Suspend then Resume from SS7 Side when SIP-T is Enabled

After a call is in the answered state, a SUS message is received from the remote SS7 side. The IMG 2020 sends the SUS message encapsulated in an INFO message to the remote SIP side.  The call is now on hold and no audio is passed. Before the T2/T6 Timer can expire a RES message which is sent from remote SS7 side is also then encapsulated in an INFO message.

The SUS and RES message are encapsulated within the SIP INFO message instead of being sent as a Re-INVITE.

Configuration

There is no configuration needed to enable the SIP Call Hold feature.