There are some numbers around where you dial a standard landline number. Then you can ring a international number from that for the same price as a national call. It may be a good idea to look into this if you're freqeuntly calling international numbers. Unfortunately I don't have any examples to hand. But maybe other members do.
To call the USA on pay as you go, if you are in the UK it's 99p per minute, if you are in the USA, it's £1.49 per minute to make a call, 99p per minute to recieve a call.