In an effort to attract iPhone users to its network, Sprint has begun offering a $100 credit when you turn in an iPhone from any other carrier.

Sprint sent us the details of the offering, which stipulates that a new iPhone must be activated by July 3 on Sprint’s network and trade-ins must be completed by August 14th. If your phone doesn’t hit the full $100 value, Sprint will match it up to $100 with an account credit, so you’ll get the full hundred bucks either way.

Beginning today, May 18, for a limited time only, customers can bring in their current iPhone from any other carrier to a Sprint store (or online and telesales) and receive at least $100 off the purchase of a new iPhone 4S (excludes upgrades) with a new line of service and two year agreement.

Sprint recently committed to buying $15B worth of iPhones from Apple, that’s around 24M units.

Sprint previously said that it needed to buy some 30.5 million iPhones over the next few years in order to stay competitive with rival carriers. The fact is that the iPhone is driving customer adoption in a huge way. T-Mobile even attributed its recent financial troubles directly to it not carrying the device, mentioning the iPhone 4S no less than 6 times in its fourth quarter 2011 earnings report.

Sprint recently posted an $863M loss in its first quarter selling iPhones, but apparently continues to believe that carrying and catering to Apple’s device is the path to success.

These are the instructions, for those interested in taking them up on the offer.

Save in store:

Reserve your iPhone 4S online at www.sprint.com (excludes upgrades) Bring your old iPhone with you to a Sprint store Save $100 instantly when you activate a new line of service!

Save Online

Go to sprint.com/iPhone to buy your iPhone 4S through our online store and open a new line of service (excludes upgrades) After you receive your new iPhone, visit sprint.com/buyback and click Trade in my device to receive a shipping label via email. (Note: If your Buyback value is less than $100, we’ll make up the difference with an additional account credit. You’ll need to activate your new phone by 7/3/12 and trade-in by 8/14/12.) Receive a credit on your bill within 2 to 3 billing cycles. Your credit may be spilt into 2 payments. Please wait the full 3 month billing cycle to receive your full $100 credit.

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