The DV adapter is simply a portable analogue phone socket that just plugs into any mains socket and connects to the hub via DECT. Any phone can be plugged into it or a DECT base station.
As described in the FAQ
No they don't have pass through but a multiway plug adapter from Screwfix is £3.19
All this of course is speculative, BT have another solution to BT ‘line only’ and the retirement of the PSTN , these customers can be made DV compatible, it’s where the ATA ( analogue telephone adapter ) , is within the exchange building and the ‘connection’ to the broadband equipment is also made within the exchange, that means the PSTN equipment still gets switched off , the building housing that equipment ( the telephone exchange) still exists, all the clever stuff is within the exchanges, the copper pair from the exchange to the customer, and everything inside the customer address is exactly as it is now , same outcome as converting the customer to broadband, just no equipment in the customer address, both solutions are on the table , which will be used is still to be decided, or still to be made public.
All this angst is for likely for nothing , as pointed out many times , line only customers are the back of the queue for this change to digital voice , to be in a state of panic now is completely unwarranted, there are two solutions, one of which the customer wouldn’t even realise they have been changed to DV .
And that is the most likely solution in the short term, until the current customer decides to get broadband, or a new owner moves in who would not be given the option of a phone only service, without it being over broadband.
A lot of worry over nothing.
Slightly off topic, but I think the media is partially to blame, whipping people into a frenzy over the changeover with their fear stories.
They could have easily done a little digging and found out about the alternate solution posted above, to reassure everybody, but they didn't.
At the moment nobody knows how DV will be supplied to telephony only customers so just wait and see what is available when the time comes. You are unnecessarily trying to solve a problem that currently doesn't exist and will probably change before it does.