Sprint's new 'iPhone Forever' plan charges monthly fee for automatic device upgrades

The plan typically costs an extra $22 per month on top of service fees, Sprint said, but makes someone eligible to upgrade to a new iPhone the moment it's released, after also trading in a previous model. Both new and existing Sprint subscribers can sign up, but the latter people must already be upgrade-eligible.

For a short time, Sprint is offering a discounted fee of $15 per month for eligible customers willing to trade in any existing smartphone for a 16-gigabyte iPhone 6. People who then upgrade to another iPhone before Dec. 31 will be able to keep the $15 rate until their next upgrade.

If a person is already in a contract with another carrier, the company is promising to pay off the older device.

The announcement comes just a few weeks before Apple's rumored September 9 press event. There the company is expected to showcase at least two new phones, commonly referred to as the iPhone 6s and 6s Plus. It could also potentially introduce a lower-cost "iPhone 6c," though if so the device may have the same specifications as the iPhone 6 in a smaller package.

All four major U.S. carriers have begun venturing away from contracts in favor of installment or upgrade programs. Sprint claims that its iPhone offering is at least $20 less per month than equivalents at competitors, although this assumes customers are picking a high-end data plan.