They will be, soon.
At the moment they can still be via a land line, but those are being phased out over the next year or so and then it will all be VOIP.
Aye our calls are via a copper cable so far but it's 'fibre to cab'* at the moment, BT are coming round to fit 'full fibre' in early January (booked a while back) All the TVs in the house have been slowly expiring over the past couple of years and the replacements have been 'smart' so connect wirelessly to the hub and we're getting short of 'bandwidth' what with those, the phone, Maz's iPad, my Chromebook and her sons Mobile it is only just coping. Main TV in the front room is still 'hardwired' to the hub via the 'BT Vision' box recorder/player. Doesn't really bother me as I don't watch much TV spending most of my time in the dining room where my QUAD lives but I do get 'buffering' issues from time to time on this chromebook.
* Maz worked at BT in the 'Network Records' office recording where all the phone lines in the country went, to which exchange etc down to which 'cab' (green cabinet) they went to and down to which individual pole each line was connected to. This was all done on paper maps but her last couple of years were spent scanning these maps to 'digitise' the system.