When you purchase a new cell phone, it typically comes with a network block on it. Your network carrier (for instance, T-Mobile or AT&T) puts this restriction on so you don't use any other network SIM within the specified period of your contract. However, you might have your reasons for wanting to switch to a different network provider. The unblocking procedure is legal and simple, but it might cost you a certain fee.
Find your phone's IMEI number (a 15-digit code). This number is usually printed on the back of your handset underneath the battery. Another way to find it out is by typing "*#06#" on the phone. This code is used for generating your particular handset's unlock code.
Obtain the unlock code. In some cases, your network provider is able to give you the code if your contract has ended or is nearing an end. They also give it to you under special circumstances; for instance, if you are travelling abroad and need to use your cell phone in a foreign country.
If you don't qualify to obtain the code from the carrier, you will have to purchase it from a website that specialises in generating unlock codes. Some of these websites include Unlock All Cellular, GSM Liberty and The Unlockers (see Resources).
Provide your carrier/unlocker company with the phone's IMEI number. If it's a third-party company, you will have to provide them with other details, such as network carrier name, phone make, model and your e-mail address. Also, make the payment via credit card. The fee depends on the network the phone is locked to, and also your handset.
Wait for the unlocking company/carrier to get back to you with the generated unique unlock code. The code is usually generated through e-mail.
Type the unlock code into the handset. For some phones, you may have to switch it on without a SIM card or with a SIM card from a network other than the one the phone is locked to. Upon recognition of the code, your phone screen will notify you that the restriction has been removed.