HTC One S T-Mobile frontStarting Wednesday, April 25, T-Mobile is giving its customers the opportunity to put their old dusty cell phones to good use. Through the carrier’s new “Magenta Deal Days” promotion, T-Mobile is offering customers between $50 and $200 in trade-in value for an old working phone, but there’s a catch: Existing customers need to be eligible for an early upgrade, while new customers are only eligible if they purchase a T-Mobile Classic or Value plan. In addition, the trade-in value only applies to folks looking to purchase a T-Mobile 4G smartphone — that includes the Samsung Galaxy S II, HTC Radar 4G and even the new HTC One S.

“T-Mobile continues to make getting a great 4G smartphone easy with our latest ‘Magenta Deal Days’ promotion,” said John Clelland, senior vice president of marketing, T-Mobile USA. “No matter how old their phone, and even if it’s valued less, T-Mobile will offer customers a guaranteed minimum trade-in value.”

Depending on the condition and model of the cell phone, T-Mobile will offer as low as $50 in guaranteed trade-in value. Still, $50 off of the HTC One S, which TechnoBuffalo liked quite a bit, is still a pretty good deal. Apple’s iPhone will pull in the most bucks (yes, you can trade in phones from other carriers), T-Mobile revealed, while Android, BlackBerry and Windows should garner $100. The sale starts tomorrow and concludes on Tuesday, May 8.