Since the Mobile device still can receive incoming calls the issue is not related to Mobile network coverage or registrations. Settings on the PBX or Mobile device are the probable cause. Perform the following checks:

Are white/blacklist restrictions active? 

E.g. not allowed to call to Premium, International, Mobile numbers?

Mobile device
Is number recognition disabled?
In case a user wants to make an anonymous outbound call he should use the Vamos App. Switching off number recognition on the mobile device itself will block the possibility to initiate outbound calls. Turn on number recognition on the Mobile device.

Corrupted registration

In some rare cases the registration of the Mobile device is corrupted. On the device itself the indicator for registration with the Mobile network is shown, however, the registration is no longer valid. The issue will be resolved by forcing a new registration.

A new registration can be best forced with the following steps:
- Go to Mobile networks > Network Selection
- Turn off ‘Automatic selection’. A list of available networks will be loaded.
- Select another Network (other then T-Mobile)
- Save the setting.
Go back to the Network Selection and turn the ‘Automatic selection’ on again, a new registration will be forced. Alternatively try to completely reboot the mobile device, this however is less certain due to caching!

In case the issue remains file a problem report.