Registration of internet telephone number ([number]) failed

Anyone get this message when they switched to digital voice? - Error code is 403
This message is transmitted by the internet telephony provider.

I'm on full fibre 300 I've raised a ticket with Zen but surprised that such a basic implementation should fail. The otherwise excellent Youtube videos from Zen suggest all you need to do is plug your phone into the Fon socket of the Fritz box (75030) and will work straight away - it didn't.