Days before T-Mobile starts selling the iPhone 5 in stores, the carrier added another incentive to lure in more customers: trade-in iPhones.

The carrier now extended its trade-in program to include older iPhones from other carriers. In other words, T-Mobile allows customers to trade in their older iPhone 4 or 4S handsets and get a new iPhone 5 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," T-Mobile marketing chief Mike Sievert explains in the press release.

In exchange, customers can get a new iPhone 5 without putting money down for the handset, and get a reduced monthly rate as well. T-Mobile is clearly making a big fuss over its iPhone offering, especially since it's a key product that was a notable absence from its lineup. Virtually every other carrier, whether small or big, started offering the iPhone, and it was crucial for T-Mobile to join the bandwagon. The carrier saw years of subscriber declines, but Apple's popular smartphone may turn things around.

The new offer aims to lure in more customers from rival carriers, or even existing T-Mobile customers with an older iPhone brought over from AT&T. A new customer would normally have to pay $99 upfront and an additional $20 per month for 24 months to cover the cost of the handset.

An AT&T customer bringing an iPhone 5 would not need to switch, but Verizon Wireless or Sprint customers might be interested in the offer, as their versions of the iPhone are not compatible with T-Mobile's network.

An iPhone 5 would net $168 in credit, or $7 in savings each month, which translates to a $13 additional charge, separate from the service fee. An iPhone 4S would equate to a credit of $120: a $15 monthly device fee on top of the service fee. The iPhone 4 is worth $24 in credit on a trade-in, equating to a $19 monthly fee.

"In all cases, you're walking out the door for less," adds Andrew Sherrard, T-Mobile's senior vice president of marketing. "Getting a real great deal for an iPhone 5."

While the new trade-in offer may not have customers flocking to T-Mobile's network, the carrier's move did catch the attention of its rivals. Virgin Mobile, for instance, is now offering $100 to any customer switching over from T-Mobile by the end of May. It's very common for carriers to offer incentives to switch over to their network, but they rarely target a specific company, as in this case.

The iPhone 5 went up for pre-order on T-Mobile last week and hits stores Friday, April 12.