HOW TO: Unlock Your CellPhone
Now that the Copyright Office has given a green light to unlock your GSM or iDen phone, you can 
save money on a new phone – provided you can find it unlocked, or unlock it yourself. Depending on your phone, you might be able to do that by punching in a code, by buying a cable, or by sending it to a professional unlocking service. This applies to Cingular, T-Mobile and Nextel phones; Sprint, Verizon and Alltel subscribers are out of luck, for reasons I’ll explain below.
If you’ve had a T-Mobile phone for 90 days, or you’ve run out of time on a Cingular contract, you can get an unlocking code just by calling your carrier. Tell your carrier’s customer service representative that you’re traveling abroad and want to use a foreign carrier’s SIM card. If they don’t give you the code, stick by your guns and ask for a manager.
If you don’t fulfill those requirements – say you’ve just bought a phone off of eBay and it turns out to be locked, or you want to use a phone locked to a foreign carrier, or you want to make a prepaid phone into postpaid – you have a bunch of different options. PC Magazine Special Report: How To Unlock Your Phone