[FS_ULBC] Support for Dual-Tone Multi-Frequency for IMS voice over NB-IoT NTN
Proposal 1: Make DTMF support part of the IMS voice service over NB-IoT NTN.
Proposal 2: Design DTMF support under the following assumptions
The DTMF packet size is at most as large as the voice packet size
Delay requirement of DTMF is not as stringent as voice service
DTMF takes priority over voice
Proposal 3: SA4 designs the mechanisms needed to support voice and DTMF multiplexing for SPS and works with RAN1 and RAN2.