GSM Providers (AT&T, T-Mobile)
Apple's iPhone uses GSM (Global System for Mobile Communications) technology to operate on a wireless network. Of the top US carriers, only AT&T and T-Mobile use this technology for their network. This makes using an iPhone on the T-Mobile network a rather simple process. Many smaller cellular providers use GSM as well (Cellular One, Einstein Wireless), but for this example we will be using T-Mobile, as they are by far the largest.Here are the steps to use your iPhone on T-Mobile.
1. Jailbreak your iPhone. This is absolutely necessary if you wish to use your iPhone on the T-Mobile network. If you don't feel it is worth the risk, then this is your chance to turn back.
2. Make sure you have T-Mobile service (obviously). Sign up for whatever plan best suits your needs. Being that you'll be using a smartphone on this plan, I would suggest unlimited data, as you'll be using a lot of it. Once you have picked your new plan you need to make sure to get a 3G SIM card for use with your phone. The SIM card is essentially the brains that your phone will use to connect to your provider's network.
3. Before placing your new SIM card in, go into the settings menu of your iPhone and click on General. Then, click on Network. On this menu, click the 3G slider to the "Off" position.
4. Insert your T-Mobile SIM card. When picking up your SIM card at your T-Mobile center, they should provide you with a small tool tool to open the SIM door on the top of your phone (as seen on the right). If they don't give you one right away, be sure to ask, as it is crucial for this step. You can leave your phone on while doing this. Once you have clicked the SIM card back into place you should notice the "Carrier" icon at the top change to T-Mobile.
5. You're almost done! Now all you need to do is alter one setting in the iPhone to make it compatible with T-Mobile's data network. Simply go into the iPhone's settings menu. Click on General, then click Network. Here, click Cellular Data Network. There should be a setting called APN under the Cellular Data title, click it. Here, type 'epc.tmobile.com'. Then, exit out to your home screen and boot up your web browser to be sure it is working properly. For other, smaller (not T-Mobile) GSM carriers you will need to find out what APN address they use for their data network. A short call to your local provider's store should yield a quick answer.
That's it! You now have a non-AT&T iPhone!