What woody2000 says, It's what I do and I pay about 4/5p per minute with a lebara international sim card, don't have to mess about finding an internet connection, just use the phone.
The sim cost me a 20 quid with 10 quids worth of credit on it from vodafone shop, cheapy hand-set cost me 20 quid. Costs a lot less than using a UK service provider as far as I can tell. Might be able to get cheaper but I can't be bothered looking as I'm happy with the service and coverage etc, used it in India, Israel, the US, Tanzania, Panama.... all over the place really without any problems, and I can top up online if needs be as well.
And for the US, if you try keepspeaking.com, they do us sim cards and will even rent you a handset for use while y'there.