The whole point of the internet is that it's international. For VOIP you can use providers from US, UK, or indeed anywhere else. Typically what happens is that they'll charge you their own local rate (which is usually a bit higher than the regular telephone local rate) for calls to the country where they're based, and a low international rate to everywhere else. If, for example, you have lots of friends/family in UK, get a UK provider. Similarly if you make lots of calls to the US, get a US provider. There are lots to choose from. For a quick overview go to
http://barorny.com/html/voip_telephony.html which gives you several links to play with.
I've been using VOIP from home for ALL international calls for nearly two years now. It's reliable, good quality, and very cheap.