Days before T-Mobile begins selling the iPhone 5 in stores, the mobile carrier is luring customers with an offer that lets them trade up from an iPhone 4 or 4S — at no cost upfront.

"Our message to iPhone 4S and iPhone 4 customers is simple: Bring in your device and trade up to iPhone 5 on T-Mobile," Mike Sievert, the company's chief marketing officer, said in a release.

The "$0 down" promotion, which continues until June 16, also comes with a credit of up to $120 (depending on the trade-in value of the device), which can be used on monthly payments, an existing T-Mobile bill, or buying another phone or accessories.

Without this offer, a T-Mobile customer would typically shell out $100, plus $20 a month over two years, to pay for the iPhone 5 (and that's in addition to a phone and data plan). The $120 credit would cut the monthly fee down to $15.

SEE ALSO: Which Carrier Offers the Best iPhone 5 Plan?

The iPhone 5 hits T-Mobile stores this Friday, although the device has been available on pre-order since last week.

Combined with its recent decision to kill wireless contracts, T-Mobile's promotion is a clear attempt to get out of its rut as the nation's fourth-largest carrier by attracting more customers.

Do you plan to take advantage of T-Mobile's trade-in offer for the iPhone 5? Tell us in the comments, below.

Image by Pete Pachal, Mashable