Will a T-Mobile SIM card work in my iPhone? They are both GSM/EDGE.
Out of the box, the GSM version of the Apple iPhone (up to the iPhone 4) and the iPhone 4S will only work on Apple approved GSM providers (i.e. AT&T).
T-Mobile SIM cards can be used in an iPhone that has been jailbroken and unlocked via third party software. There are a variety of commercial and non-commercial software options available which unlock the iPhone.
Though unlocking has become very common for users that do not want to be tied to a particular carrier or bound by a contract, users should understand that unlocking is performed 'at your own risk'.
For more info on jailbreaking, including HOWTOs, tips, articles about tweaks and jailbreak apps and more, visit our iPhone Jailbreaking section.