It's really very simple to setup, companies like AAISP offer a pre-configured solution. As a example AAISP is £1.20 a month + calls and on average the monthly bill is about £2.50 P/M rather than the £40 BT where charging.
Yes both account offer 2FA protection.
AAISP -
https://www.aa.net.uk/voice-and-mobile/voip-information/
Sipgate -
https://www.sipgate.co.uk/
Sipgate offer a number of solutions from PAYG to PM, they also good setup guide for a lot of VoIP hardware which is very easy to follow.
As an example for hardware, I use a Gigaset N510 Pro Base with Gigaset SL800H PRO and S700H PRO handset but you can also get gateways like the Cisco SPA112 if you want to keep your old phones etc..