I did something similar. I wanted the cheapest mobile I could get, but found it wouldn't take my old sim card. I tried a couple of websites, but they couldn't unlock it. It's legal to unlock a mobile, but the manufacturers often don't make it very easy. Calls to the help desks usually result in fobbing off or attempts to persuade you to stay with them. Getting the number transferred is a pain. In the end, I just did what I should have done to start with: I took it to a stall in the market. It cost me £8, but if you're prepared to haggle you could get it cheaper, no doubt. Some mobiles can be unlocked by inputting a special code, but others require a special lead and equipment.