I use Skype to Go. You get a local number to call from anywhere (cell phone, any land line) and you access your Skype service. Check it out!

I only use AT&T like you said it costs pennies anyway to call the UK.Phone cards are usually cut in half anyway you call the UK

I agree 100%. We may $10 a month and via the PC that gives us unlimited landline calls anywhere in the world.
You also get a US local number (kinda like a calling card number) from which you can then call anywhere if you prefer to use a cellphone or home phone. Excellent quality and video calling also if you want it.

I use Voipcheap to call the UK and Europe. It works well for me, I make a lot of overseas calls. I pay €10 for 90 days. From that €10 each time you connect 5 cents is deducted, and you can call for unlimited time. The calls are connected through Voip to the phone number you specify and then it just connects your desired number. You are using a proper phone which I find very useful and because some of my calls are over an hour in duration it is very cost effective.

http://www.voipcheap.com/en/index.html