Well,if you want to use voipcheap over in the states,I have been using it for a week over here in seattle for a week with no problems.Just dial a UK number and thats it! Dials faster actually than in the UK.
I use a b2k voip box and voipbustermate (latest version) with no problems. Clear this way,the other side say its not a very good line sometimes,but its clear enough this end.