You can get a cable to unlock a mobile phon using your PC.
I think it costs about $15 or so from ebay.
When you unlock it you can use any network/sim card on it, most of my phones are unlocked.
As Anjay said, you normally have to pay a fortune in a shop to get an unlocked phone so get one which is locked to the one network and unlock it yourself.
A lot of places in England also unlock phone for you which costs around the same as the cable itself.
Also, some phones dont work in some countries, I have two that I thought worked in the stes but neither do..
Some are dual-band meaning they work in europe and asia, think also africa. And some are triband which allows them to work in the states.
Hope this also helps
PS. Another quality post from Anjay, good to see you coming back and still posting your great info