I ordered full fibre max. 5 mins later zen phoned me back and said because I have digital voice I would lose if I upgraded because it's not compatible with the new router. I said that's ok I've been using my own hardware anyway. The Zen operative checked with his supervisor but said I would still not be able to keep digital voice. I said what difference can the speed make to the SIP server they use for digital voice. He then said my only option is to move my number to a VoIP provider and then upgrade. I'm sure this can't be right.
Whomever it was you spoke to is a very confused bunny. It certainly is possible to port a number that is currently hosted by BT (Digital Voice). Number portability is mandated by OFCOM. How that number is hosted / platform or technology is irrelevant.
Of course porting will apply a cease request to all other bundled products (broadband etc) that form part of the same BT service agreement. But that's not the point.
So no, not right at all.