+1 for Voipfone. Stellar service for the five years since I had the BT number ported and cheap as chips, too. When we moved abroad for four years I switched to an app on each of my and my wife's smartphones and paid the small premium for each to be treated as an extension; an incoming call rings both phones and wheover gets there first stops the other one ringing. If neither answers and a message is left, it is emailed to us. Outgoing calls are cheap globally, and to the UK remained free (included) as the internet is used to connect to the Voipfone servers so it doesn't matter where you are. You always appear to be in the UK.
It works so well that now back in the UK, we haven't reverted to a landline. Our UK 'geographical' home phone number remains on our mobile phones. Yes, I know there is a small emergency risk in not having a phoneline powered from the exchange, but with two handsets and three network service providers, we take our chance.