I had bought an iPhone 5s and there was still the old persons phone number any way I can remove it?

I had bought an iPhone 5s off craigslist and it still had a number on it so I couldn't switch it to my number I can't get a hold of the guy I bought it from either. Any help?

More specifics needed, like if it is GSM or CDMA.

Reset it back to factory specs and set up like a new iPhone. That erases (almost) everything. But if you did that and still see the guy's name/number then most likely it's been reported lost/stolen. Notice you can't a hold of the guy who sold it to you can you? Lol. It's called iCloud activation lock. Unless that old guy logs into his iCloud and removes his information and password from the phone (you can't) then you got scammed. Apple will not help and doesn't care. They don't do third party transactions or drama. Better give old guy a call. Best luck.
http://support.apple.com/.../kb/ht5818