Calling internationally with Facetime on an iPhone

I just got an iPhone 4S on Saturday, so forgive me if this is a n00b question. How much would it cost, if anything, to make an international call with Facetime? I live in the US and a friend of mine lives in the UK. We both have iPhones and we can text with iMessage, but I wasn't sure if it'd also be free to use Facetime to do it as well.