T-Mobile has a nice little spring cleaning deal starting tomorrow. From April 25th until May 8th they’ll be offering the Magenta Deal Days promotion, recommending users trade in their old phones, slower smartphones, or even those slow 3G iPhone’s for a brand new 4G smartphone from T-Mobile. Full details and cash value listings below including how the iPhone will get you an HTC One S completely free.

T-Mobile is offering $50 dollars towards any 4G smartphone if you trade in whatever old phone you’re using or have laying in the closet. Even better is the smartphone deal. They’ll give you at least $100 for any Windows, Android, BlackBerry or Symbian device and even more if you toss in that old iPhone of yours. Here’s an easy guide courtesy of T-Mobile:

They’ll take whatever old iPhone you have and give you $200 towards a brand new 4G phone like the extremely impressive HTC One S available tomorrow. In case you forgot the One S will be available tomorrow for $199, making it completely free if you trade in that old iPhone and sign a new 2-year contract with T-Mobile. Be sure to read our HTC One S review so you know you’ll be getting a 1.5 GHz dual-core processor, and an awesome 4.3″ qHD screen if you decide to bite on this deal.

T-Mobile has multiple impressive 4G phones like the upcoming One S, the HTC Amaze 4G, and even the Galaxy S II so you’ll want to head to your nearest store for all the details – or feel free to just click here. Who will be getting the HTC One S or one of the phones listed below tomorrow?

Update: We reached out to T-Mobile to clarify how this all works and this was what we received in return. “Functioning eligible smartphone and 2-yr agreement on qualifying plan required.” So as long as that old beat up iPhone still works take her in and get a brand new T-Mobile phone guys.