When configuring the fritz,
Select the following:
'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 on mPBX,
or if using SIP Device then the device details must be entered here.
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.