I use
vonage for all my out going calls. It uses my normal phone so you just pick it up and use it like normal. Costs £7.99 + VAT a month for all UK calls and some overseas as well ( not bothered which because I don't call abroad.).
I have kept my BT line for incomming calls as I don't want to loose my number and I cant port it over to vonage yet. As soon as I can it will be bye bye BT and I will save the £9.99 a month for the phone line.
We have been using vonage for about 6 weeks and it is very good. The only problem is the phone is a little too loud. I will phone tech support about it sometime.
I have a 2 line to one phone adapter connected so I only have 1 phone with the 2 lines. Either line ringing will cause the phone to ring but when you pick up the phone to dial out it always uses the Vonage line.