Why does my Softphone Mobile only work with one of the SIM cards if I have more than one SIM card?

Problem:

The user tries to start a call via the Softphone Mobile using a specific mobile network. However, this is terminated immediately after dialing. In the Wireshark trace of the XCC, the client does not show any candidates in the SDP header and the call is terminated before the session progress.

Cause:

The user has two SIM cards in his device a primary and a secondary one. Only the phone number of the secondary SIM card has been entered for the user in the XPhone Connect Server. If a call is now initiated the STUN request is sent to the STUN server via the primary SIM card and the call is set up via the secondary SIM. As a result, the client does not receive any candidates from the STUN server for the secondary SIM card and the call is terminated.

Solution:

The only way to avoid this behavior is to use the SIM card with the phone number entered in the XPhone server as the primary SIM card. As a result, the client should also receive the corresponding candidates from the STUN server and the call can be established.