Figure 7.7.2-1 illustrates a high level call flow for an interworking scenario between a UE and a PSAP via the CS domain.
This scenario may occur in the case, when PS access is available, but the UE does not detect that the NG-eCall is supported by the IP-CAN and there is no CS access available. In this case, the UE shall establish a regular IMS emergency call.
A similar scenario may occur in the case, when the UE detects that the IP_C AN supports NG-eCall and initiates an NG eCall but the session is handled by a PSAP not supporting NG-eCall. The only exception would be step 2 and where the SIP INVITE shall contain the initial MSD and the eCall type of emergency service indicator (automatic, manual).
The UE performs steps 1 to 6 of the procedure shown in Figure 7.1.
The UE sends an initial emergency SIP INVITE to the IMS core. The SIP INVITE shall be setup as follows:
if the UE has not received the "eCall supported" indication, the UE shall include the eCall type of emergency service indication (automatic, manual) and shall not include the initial MSD if the PS access is available;
if the UE has received the "eCall supported" indication, the UE shall include the initial MSD and the eCall type of emergency service indicator (automatic, manual).
The IMS core routes the emergency SIP INVITE towards the appropriate PSAP. In this call flow, the appropriate PSAP is accessed over the MGCF and the CS domain after translating the eCall type of emergency service indication into the corresponding MSISDN.
The SIP INVITE is sent to an MGCF/MGW for interfacing to the CS domain.
The MGCF sends an IAM towards the appropriate PSAP for the emergency voice call.
The emergency voice call establishment is completed with a voice path only.
If the UE has sent the MSD in step 2 and did not receive acknowledgement for the initial MSD included in the SIP INVITE, or the UE did not send the MSD in step 2 the UE shall attempt to transfer the MSD to the PSAP via the eCall Inband Modem, as defined in TS 26.267