A call can be established by an ICS UE that supports Gm, but without using the Gm reference point (for example, as a result of CSFB, or when in non-DTM GERAN coverage). If Gm is available after call establishment, or becomes available (for example, as a result of a change of access network due to handover), no mid-call services are applicable to the call, and the call is anchored in the SCC AS as described in clause 126.96.36.199
or 188.8.131.52, the UE may augment the ongoing CS call in order to be able to use Gm for service control signalling.
If the UE does augment the ongoing CS call with Gm service control signalling then the UE shall use Gm when establishing additional calls, or for any mid-call signalling. The UE shall continue to use Gm procedures until the existing calls end.
If the UE does not augment the ongoing CS call with Gm service control signalling then the UE shall use CS procedures when establishing additional calls, or for any mid-call signalling. The UE shall continue to use CS procedures until the existing calls end.
If configured to do so the ICS UE may invoke IMS signalling to add Gm service control to the ongoing session.
When the UE determines a need to augment a CS call with Gm, the UE initiates registration with IMS via the new PS access (if not already registered). It initiates the augmentation by sending an INVITE to establish the service control signalling via the PS access. The INVITE includes the SDP for speech or speech and video and non-real-time media flow(s) and the static (provisioned) STI, as defined in TS 23.237
Standard procedures are used at S-CSCF for routing of the INVITE to the SCC AS.
The SCC AS identifies the session to be augmented using the STI. If the INVITE request contains additional PS media the SCC AS also performs the Remote Leg Update to update the combined session towards the Remote Leg.