I have used 3CX which is a VOIP based phone system for years connecting to the PSTN network via an analogue gateway. Now that the PSTN network is being switched off is there a SIP server I can log into via the internet rather than utilising my analogue gateway plugged into my HUB3 router. I'm effectively going from digital to analogue back to digital - would be good to ditch the analogue step.
Companies like Andrews & Arnold or Sipgate are worth a look.
Naturally there are many others depending on your service needs / budget.
We ported 2 BT lines to Sipgate in 2017 and once configured it has been fuss free and inexpensive.