BT helpfully sent us a card to tell us how how set up our new home phone service. In fact, they sent three:
(address redacted for address-redaction reasons).
These were sent separately through Royal Mail, so represent a considerable expenditure, what with the cost of stamps these days.
The instructions on the back basically said "disconnect your old phone from the wall socket and plug it into the back of your router in the socket marked 'phone'.", although it didn't mention you'd have to wait a day for it to work.
We now have our phone live again, and have already received our first silent call checking that we can be added to the list of numbers that hoaxers can try.