Along with adding the iPhone 5 to its device lineup this week, T-Mobile has announced a new trade-in program for iPhone users. Under normal circumstances, T-Mobile will be selling the iPhone 5 unlocked for its full price or, alternatively, $99 upfront with two years of $20 monthly payments, which works out to $579 after 24 months – a smooth $70 savings over buying the unlocked handset directly from Apple.

When the carrier begins offering that option on Friday, it will also allow people with older versions of the phone to turn in their device and get the latest model with no upfront fees as well as a discount on their monthly device bill. In other words, you can hand T-Mobile your used iPhone 4S, get the iPhone 5 sans $99 charge plus $120 in credit toward the device, which pushes the monthly fee down to only $15.

T-Mobile will also waive the $99 fee for iPhone 4 owners, though they'll only get $24 in additional credit or $1 off per month, according to CNET. Meanwhile, folks who already own a Verizon or Sprint iPhone 5 – which are incompatible with T-Mobile's network – will receive a discount for switching. Again, the initial $99 fee is dropped along with $168 in credit, which chops the monthly charge from $20 to $13.

T-Mobile opened preorders for the iPhone 5 on April 5 and it officially launches on April 12. The carrier is clearly excited about finally being able to offer the device, which will presumably play a key role in the company's "uncarrier" initiative – a new marketing effort to depict T-Mobile as being a consumer-friendly alternative to its larger rivals. Or as the company itself says: "We're still a wireless carrier, we're just not going to act like one anymore. From now on, many of the things our competitors do, we don't."