You can call any country in the world so long as you have the money and the country code. All US telephone numbers have to be prefixed with a 001 before you dial and the UK needs 0033.
A phone takes its number whichever country it is in so if you wanted to call (l'm British and so is my phone number) but I was on holiday in Nigeria you still need to use the 0033 in front of my number and not the Nigerian code.