I had BIG problems when BT turned me over to digital voice phone. Although I had got the newest smart hub my existing phones refused to work with it. An engineer came and connected them up but within minutes of his leaving the service was down again. In the end my nephew got it working by actually editing the software of the hub and the phones. However half of the facilities on the old phones have disappeared. I had to buy two new handsets guaranteed to work with new smart hub.
There was nothing whatsoever wrong with the old phones and they still work in a limited capacity but some of the advanced features are missing and we could not find a solutiuon. Eventually gave the old phones (a base and 3 hand sets) to my cleaner who is not bothered about the advanced facilities not working.
I like the ability to block international calls, to create a White list of approved callers, and so on. However the new phones do have some good advanced facilities like the ability to take two calls at once, divert to/from a mobile, and other things I havnt explored yet. The new phones are also suposed to work with Alexa but we havnt had any success with that yet.