I found an Iphone 4 how can I remove the original number from the phone?

I want to sell it, as I'm an Android guy myself and don't have much respect for Apple products. I want to unlink it with their sprint account. I already did a reset and tried the DFU mode. Their number is still listed in the about.

Added (1). I can't return it because the person hasn't filed a report about it so it was obviously not that important to them it was obviously left by a rich kid who is capable of getting a new one i looked up at my work no on reported it so they don't want it. And the nearest Apple store is 45 mins away. I'm not spending that much gas to return it.

Go to the apple store

Give it back. It's not yours.

Give it back to whoever lost it. I'm sure if you lost your phone you wouldn't appreciate some person trying to sell it to some other guy. Be a good person and find whoever owns it. You'd want the same done to you.