Most of the time when you purchase an iPhone it comes with a contract with a designated service provider. When a user has completed the terms of the contract they may want to use the same iPhone only with another provider. For this you will have to know how to unlock iPhone 3GS for T-Mobile. After it is unlocked it can be used with any cell service provider.
The easiest way to unlock the cell phone is to call the network and speak to one of their operators. Ask them for the unlock code for your phone. They will verify that the phone is yours before giving you further instructions. The instructions may be different depending on what network and operator you are speaking with. Usually, they will send this information through your email. This makes it easy to follow and unlock the phone.
To switch to T-Mobile first turn the iPhone off. Using the proper tool remove the SIM card carefully. Then replace it with the SIM card that was supplied by T-Mobile. Make sure that the tray is closed properly before proceeding. Then turn the iPhone back on. When it reboots it should automatically connect to T-Mobile’s network.