According to the carrier's www.wmdeals.com site, T-Mobile is offering the HTC Radar 4G for free when you order the device online or by phone. The $35 activation free is waived and the phone is sent to you second day air at no additional charge. You need to sign up for a two-year Individual or Family Classic Unlimited Plus Plan. Whether phoning in your order or ordering the device online, you need to mention the promo code "WINDOWS PHONE". Although the phone is free, you still need to lay out $100, which you get back in the form of a rebate.
That is only half of the story. Now that you know how to get the HTC Radar 4G for Free, there is still the matter of getting your $100 profit. To do that, you need to head over to T-Mobile's trade-in site at mytradeins.com. Enter the promo code "MSMANGO" and you will receive at least $100 for any trade in no matter how old it is, as long as it is in working condition.
So there it is. Thanks to a couple of promo codes you can buy the HTC Radar 4G for a net price of zero and get $100 for any ancient handset that still works. Right now, Wirefly is offering the phone for $49.99 with a 2-year contract, but that doesn't even come close to picking up the phone and getting paid $100 to do so.
source: T-Mobile (1), (2)
Get an HTC Radar 4G for free after a rebate and your signature on a 2-year contract (L), and then trade in any old working phone for $100 (R)