Two comments.
We went to digital phone ( ie our land line via the internet router) in
early Oct. The switch was faultless. At the time, we had 50M internet via
Fibre to Cabinet.
We opted to keep our existing phones - simply due to having a previously
bought a set of Dect phones we like and Senior Management having a
particular retro style phone on her desk she likes. I just plugged the
existing wiring into the BT supplied box which links to the router ( it is
a DECT converter as far as I can tell).
We have since gone to Fibre to the House (900M) - again without problems.
If you can get a Fibre service ( Fibre to Cabinet or House), I would
recommend it.
True, if there is a power cut you can’t make calls. You need to decide if
that is a factor. In our case, we have mobile coverage.
Second comment.
I stumbled across something yesterday which I have yet to try but, based on
research so far, I think will work. I saw it on YouTube.
I have HUAWEI 4G router - the type some Mobile companies supply / supplied
for home broadband. I use it in our motor home and when travelling
generally. It has a RJ11 socket which I assumed was for phone line based
WAN, like the BT hubs. I had never used the socket.
It seems it is actually a phone socket. If you plug in a ‘normal’ phone (
the kind you would normally plug in at home BUT with an RJ11 plug, not the
BT special and the SIM in the router isn’t Data only, you can make /
receive calls. The only other thing you need to do is set Network
selection to Auto.
I’m not aware of any way to transfer a landline number to a mobile.
Of course, you need to find a deal from a mobile company ( or one of their
secondary sellers) which covers your area.