They won't have to 'enter' every house.
The voice service is likely to be from an ATA either within future HUBs or by a plug in device ( Like those already on the market). The customer will just plug in their phone to the 'phone socket on the ATA/Hub.
The ATA is just a piece of software controlling the Phone socket and setting up a VLAN ( 144 to 168Kb) at CBR ( Constant bit rate) across the network. VOIP call servers are cheap.
Using an MSAN is expensive compared to this which is why BT stopped rolling out the Voice 21CN in 2009. Just the MSAN management is expensive and you would need combination cards to handle the Voice and BB which would reduce the capacity and increase the cost. The existing cards are BB only.
CP voice capacity is not irrelevant, they may still be able to buy wholesale from BT All the full LLU suppliers have their own voice supply (
4.4m WLR and 5.53m full LLU @ April 2018)
Upgrade will be over time with the only BT led part being those PSTN only customers where they will have to provide the BB service and the Hub, The BB only being active for the Voice VLAN. ( Could be over the oldest DSL type as only need less than 500kb).
BT would also be looking to come out of the regulatory straightjacket of the PSTN world which costs a lot on the management side.
This type of product is already the only choice at FTTP only sites which will be all new builds well before this date, I can foresee BT not providing copper service to new multi dwelling developments from around 2020/22 and am surprised people cannot see this coming.