Though AT&T is the only official service provider for Apple's iPhone, if you "unlock" your iPhone, which entails downloading an application onto it that allows you to use it with any carrier, you can choose to use it with another wireless service provider, including MetroPCS, that offers contract-free service. The nature of the tasks involved is simple, although completing all of them will require diligence on your part.
Unlock your iPhone. Point your browser to blackra1n.com and download the software package that corresponds to your computer's operating system. Install the software onto your computer and run it once the installation process has completed. Connect your iPhone to your computer using the USB cable that came with it and click the "Make It Rain" button within "blackra1n." Wait a few moments for "blackra1n" to install itself onto your iPhone, which will cause your iPhone to power off and restart. Tap the "blackra1n" icon on your iPhone and then the "Sn0w" icon within "blackra1n"--doing so will unlock your iPhone so that it can be used with any carrier.
Take your iPhone to a MetroPCS store to "flash" it so that it can work on the MetroPCS network. The "MetroFLASH" procedure costs $10 per phone as of October 2010.
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Sign up for a MetroPCS account if you have not already done so. Speak with the associate at the store when you take your phone to get flashed to begin the process.
Transfer existing MetroPCS service to your iPhone by calling MetroPCS Customer Service at 1-888-863-8768. Turn off your old MetroPCS phone before beginning the process, as it will transmit signal to the MetroPCS tower and disrupt the transfer process. Inform the agent that you want to transfer service, making sure to keep your iPhone's IMEI serial number--a 15-digit number you can find in the "About" tab of the "Settings" menu--on hand, as the representative will need this to transfer your service.