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.