When configuring a fritz!Box PBX select the following from inside the web interface of the fritz PBX:

'Internet Phone'

'SIP Trunk' option if client has a trunk, otherwise standard VoIP/Internet Phone option.

Telephone number for registration should be their DID including area code.

Username, Password and Registrar information can be taken from the Trunk options from your PBX portal.

Country code is not required, should be left as None.
Area code should be set to (0X) Where X matches the area code on their DID.

No other settings here require configuration.

This should now register with no further issues.