Content for  TS 23.172  Word version:  18.0.0

At any given time, if either of the call parties wants to change from the current active mode to the other mode via MMI, the terminal shall activate an In-Call Modification procedure. Using this procedure, described in clause 5.3.4.3 of TS 24.008, the UE shall send a MODIFY message containing the BC-IE to change to. This BC-IE shall be one of those already negotiated at call setup. As a result, the MSC shall then invoke the service change procedure (see clause 4.3.5). If the request is accepted, the originating MSC sends a MODIFY COMPLETE message to the UE including the BC-IE of the mode to switch to (see Figure 4.13). If the request is rejected, the MSC sends a MODIFY REJECT message to the UE including the BC-IE from the current active mode (see Figure 4.14). In the case the MSC has determined that the other mode is unavailable (e.g. a fallback to either mode has occurred), it shall reject the MODIFY request at once by replying with a MODIFY REJECT message. On the remote side, the MSC shall initiate an In-Call Modification procedure towards the terminal using the MODIFY message. For a service change from speech to multimedia, the terminal shall request confirmation from the user unless configured differently. For a service change from multimedia to speech, the terminal shall accept the request without requesting confirmation from the user. If the change is accepted, the UE shall reply to the MSC with a MODIFY COMPLETE message, whereas a MODIFY REJECT message shall be sent if the change is rejected. When receiving a MODIFY message, a terminal shall not interrupt the data traffic and shall continue to send and receive data in the old mode, even after the terminal accepts the modification with a MODIFY COMPLETE message. The radio bearer will then be reconfigured by the network. After the radio bearer has been reconfigured, the terminal shall send and receive data in the new mode.