Yip, thanks Kris_: The contract will be with VM but I mentioned Nexfibre to be distinct from VMs normal offerings. So I think you and I are in a very similar position. VM don't seem to offer any telephony alongside the Nexfibre (XGS-PON) service. I am moving from a FTTC broadband and telephone (PSTN/copper) service so potentially lose my longstanding telephone number which is used to call me by elderly relys.
So I'm looking to see what if anything others have done with a similar move. e.g. Can I port my number onto someone's SIP offering? Just looking at Options if I effectively lose landline telephony in the move to VM over Nexfibre.
Having now read the responses to your question posted on VoIP for Beginners at
https://forums.thinkbroadband.com/general/4749250-voip-for-beginners.html#Post4749250 on thinkbroadband I can see the various SIP provider options and that I can potentially pay BT/OR a one off porting charge.
So I am off to do further research on SIP providers.