23.1723GPPRelease 17Stage 2Technical realization of Circuit Switched (CS) multimedia service UDI/RDI fallback and service modificationTS
If the preferred mode, that is the first BC-IE indicated by the originating UE, was selected as the result of negotiations, the call shall be set up normally towards the originating UE.
If the negotiation resulted in a change of the selected mode, i.e. the call was set up as "multimedia first" and changed during the negotiation to a speech call, or vice-versa, because of either fallback or change of selected mode and the Call Proceeding message has been sent then an In-Call Modification procedure (see 3GPP TS 24.008 , clause 126.96.36.199) shall be initiated towards the originating UE after the call control entity has entered the active state, i.e. the CONNECT message has been sent.
Figure 4.10: Preferred mode selected
Figure 4.11: Less preferred mode selected, accepted
Figure 4.12: Less preferred mode selected, rejected
If the Call Proceeding message is delayed until response from the terminating side then it may include one or both BCs either in the order requested from the Originating UE or the order of the BCs may be reversed, depending on the result from the negotiation or (single BC) due to fallback. See Figure 4.12a.
Figure 4.12a: Call Proceeding Delayed