Get a T-Mobile PAYG sim and pop it into an unlocked tri-band GSM phone. I use an old Nokia that used to be on Orange.

Then set up an account with jajah

http://www.jajah.com/

Other services maybe available, but this is the one I use.

Save the phone numbers you use most on jajah. Register your US PAYG mobile number and then save the jajah direct local access number into your mobile.

Boom - you pay a couple of cents a minute to T-Mobile for the local call, and then a further 2p a minute to a UK landline or 13p a minute to a UK mobile.

If there's a cheaper way to call home from a mobile in the US then I haven't found it.

This also gets around having to fanny around with calling cards and the like.