Unlocking a phone enables it to be used on just about any network. If you have a T-Mobile supplied Samsung phone, there are several codes that can be used to unlock it. The only restriction is that these codes can only unlock Samsung phones that use SIM cards for activation, so it will not unlock Verizon or AT&T phones. Once the handset is unlocked you can start to use it on all SIM networks immediately.
Remove the back cover and battery from your T-Mobile Samsung handset.
Write down the number on the sticker next to the initials "IMEI." IMEI stands for International Mobile Equipment Identity, this number is unique to your handset, so do not substitute it with an IMEI number from another phone.
Remove the Subscriber Identity Module (SIM) card from the SIM card cradle. The SIM card is a small rectangular shaped chip that is used to service your phone. Once you remove this card, write down the number printed on the front of it.
Go to a site that generates unlock codes. Enter your phone's make and model.
Replace the battery without the SIM card.
Enter the IMEI number when prompted, and if necessary your SIM card number.
Write down the code you are given.
Power your phone on and once the phrase "Insert SIM" or "SIM Card not recognised" pops-up on the display screen, enter the code. If the code was entered correctly, you should get a message saying, "SIM Restriction Off."
If you are unable to find a code particular to your phone, or one that works, you can send your phone off to a company that can unlock it for you. They will charge a small fee for this service and may even charge postage. Once your handset is unlocked it will work on most GSM networks. All you'll need is a SIM card from the company of your choice.