If you're looking to buy a new iPhone this month, T-Mobile says it will provide mobile customers with better trade-in values than rivals like AT&T, Sprint, and Verizon.

T-Mobile is "guaranteeing we'll give customers what they deserve—the best value in the industry," said CEO John Legere.

To make good on its promise, T-Mobile intends to track the market every day to see what competitors are charging. And, if a customer can find a better trade-in offer from a major national carrier, T-Mobile will not only beat it, but also give them the difference, plus another $50.

"When that new superphone's price is the same everywhere you go, the difference in your final cost comes down to the trade-in value of your old phone," Mike Sievert, chief marketing officer at T-Mobile, said. "Now when you're ready to upgrade—it's a no-brainer where to find the best value—the Un-carrier. Guaranteed."

The offer begins Sept. 17 and lasts only a limited time, though T-Mobile did not provide an end date.

T-Mobile will likely land the new iPhone, which will be unveiled tomorrow and likely hit stores later in the month. The new Sony Xperia Z3 is also coming to T-Mobile, as are the new Galaxy Note 4 and Note Edge phablets.

T-Mobile has been making headlines since June, when PCMag's Fastest Mobile Networks study crowned the company "most improved" over 2013.

But the carrier has also attacked its rivals via data-heavy family plans, rescue programs, more mobile and tablet data, and streaming music that doesn't hog data.

For more, check out Where to Sell Your iPhone for the Highest Price.