Change the operator to one that implements SIP.
You could try moving to PJSIP, as it appears to have some features to work round this sort of broken SIP implementation: PJSIP: Change the user part of the Contact Header
Change the operator to one that implements SIP.
You could try moving to PJSIP, as it appears to have some features to work round this sort of broken SIP implementation: PJSIP: Change the user part of the Contact Header