T48G problem with incoming calls

Hi together,
Since 2 days I have a problem with my T48G. Each incoming call will be rejected, the test via the SIP online portal (vendor: sipgate) reports a connection error and the T48G reports a new missed call. The SIP vendor's technical support has also checked the connection an means, that the phone does not accept the incoming call.
Outgoing calls are ok. I have reset all settings und have updated the firmware - but no success, the failure is still remaining.
The time before the 2 days all was working fine...

Any ideas what I can do?
I like the phone an I will not replace it, but I need it working.

Thanks, Alexander