Report of sa wg2 meeting #114

Discussion and conclusion:

This was postponed to SA WG2 meeting #114.

TD S2 160027 LS from CT WG1: LS on failure to set up the resources for VoLTE MO call. (CT WG1)
Abstract: CT WG1 thanks SA WG2 for their LS on failure to setup the resources for VoLTE MO call. CT WG1 has discussed the scenario described by SA WG2 and reached the following conclusions:
- UE retry is only applicable to MO calls, not MT calls
- In case of IMS emergency call setup failure, SIP 380 (Alternative Service) response is used and the UE
retry behaviour upon receiving this response is already specified in TS 24.229 and TS 23.167.
- In case of normal (non-emergency) IMS MO call setup failure, in deployed networks different SIP
response codes are used by different operators to trigger various UE retry behaviours.
In certain cases it can be useful to give the UE an indication that the network has failed to setup the resources for the VoLTE call, for instance using a SIP 500 response, so that the UE could use this information to decide whether to retry the MO call in the CS domain.
CT WG1 has agreed CR C1-154881 to TS 24.229 to capture this. However care must be taken such that the SIP 500 response did not get generated from the remote UE.
Therefore the UE would need to check the P-Asserted-Identity header field to check that the SIP 500 response was generated from the P-CSCF.
Action: CT WG1 asks SA WG2 group to take the above information into account.