It may not suit the OP but it may suit others in a similar situation.
In cirtain circumstances you can end a contract early without penalty. - When you have a contract with an ISP you can exit early, (without penalty), under cirtain circumstances. - (eg. if the ISP does certain things. - (eg. BT always have price rises as part of their business model but if the price rises are outside of the agreed/publish criterion that was agreed to when the customer signed up the customer can migrate without penalty).
Also, Since the change to BT Digital Voice forces the Customer to use the BT Router and his own, (eg Billion Router), works better on his line the change is not what he signed up to.
The change from PSTN to VOIP is coming by 2025 but if BT want to bring it in mid-contact/early on a customers line to suit their commercial decisions in my view the customer should be able to leave early without penalty as that is a significant change and it seems to me that BT has already recognised this by allowing customers to go broadband only when they introduce this change;
https://www.bt.com/help/landline/digital-voice-migra...
Therefore, according to the above guideance, BT will allow the customer to choose a different broadband only contract instead of the present contract that BT plan to unilaterally change.
According to BT, (as per the above link), If the customer chooses not to take up Digital Voice for their own reasons they can move to a broadband only contract.
Anyone can open one or more VOIP accounts tomorow and a new number is the easiest way, (as we all know that porting a bundled number can end the broadband).
If a person decides to not to take up digital voice and chooses the option of a broadband only contract the BT link between the new broadband only contract no longer applies. - So far so good.
If the customer just keeps the new local VOIP number that they chose when they signed up to VOIP; all is good.
If within 30 days the customer then wants to port his old/disused number to VOIP that may create an issue but theoretically since the new BT contract is broadband only it should not cease the broadband.
When I had both broadband and a PSTN phone line with Zen; they were started at different times and I ensured that they were not bundled. - When I opened a VOIP line with a new number I ended my PSTN number with Zen but even though they were never bundled the Zen system still referred to the old PSTN number, that has not existed for over a year, and I suspect that if I had tried to port the number it would have caused a lot of issues.
If the PSTN line and the broadband is not linked the theory is sound, however, since BT customer service people will not be up to speed keeping the new VOIP number is much easier if that is possible.