Bea's correct-- there might be some third party way of doing it, but let me tell you a story.
Once upon a time, I switched from T-Mobile to AT&T. But I had a Sidekick, and I was in love with the d***ed thing. So, I decided I was just going to get it unlocked, pop my SIM card in, and use it; they're on a similar network style, so I thought it'd work. Well, I could text. And I could make calls. And that was about it. And that was with two companies that are on the same kind of network. Sprint and AT&T use completely different somethings or others; I can't remember the name for the life of me, or on what the networks use. But I really don't think that, even if it was unlocked, it would work.
And even if it did, why have an expensive iPhone if you can't use the internet on it? You wouldn't be able to do much of anything.
THERE we go. Thank you! I couldn't remember what they were called for anything.
But no. I really don't think, since they're on different networks, that you'd be able to use them even for calling and texting like I did my Sidekick. But, Sprint does have its Buy Back program, or you could see what you could get for it on eBay or Craigslist.
Beyond that we have decided to only activate devices that we have tested and have agreed to accept from the manufacturer. In any case even if you adapted another carrier's phone to work on our network, we could not activate it because we do not have the Electronic serial number for that device in our billing system, and it cannot be added.
This unfortunately the correct answer is above. Tthe technology of how the two carriers handle the signals is very different. It is like trying to listen to an am station on an fm only radio But if you would like to move to sprint you can get the Iphone 4 for about 99.00 with a new account
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.