How to Configure 3G on a T-Mobile iPhone
- 1). Purchase a T-Mobile SIM card, and activate it. SIM cards are sold at T-Mobile stores and online. You activate it over the phone or online, but make sure to select a smartphone plan.
- 2). Insert the end of a paper clip in the tiny hole in the SIM tray to release it. (The tray is located on the right side of the iPhone 4 and the top of the original, 3G and 3GS.)
- 3). Put the T-Mobile SIM card in the tray, and put it back in.
- 4). Turn on the iPhone, and press "Settings."
- 5). Tap "Off" next to "3G" on the "Network" page.
- 6). Tap "Cellular Data" on the "Network" page. Type "epc.tmobile.com" in the APN field, but leave the username and password blank. This will allow you to use the T-Mobile network rather than AT&T.
- 7). Press the "Home" button under the screen to exit "Settings."
Source...