Okay guys I tried all these and nothing worked for me.
My imessage only worked on wifi.
So I called sprint and this worked for me!!!
Go to Settings>General>Reset>Reset Network Settings
Then your phone will turn off and back on
Once its back on type in this in the keypad ##273283# then dial and it should say "Starting Service Update" and then you hit "OK". Wait anywhere from 30 seconds to a minute and a message should pop up that says "Service Update Complete".
If it fails, turns your phone on and off and try it again.
This worked for me on the second try. It took about 2 minutes to kick on. Make sure the wifi is turned off once you check it. Check it by going to a internet site and texting another imessage user by making sure it's blue and not green.
Hope this helps! 🙂
Btw I live in the Ohio (USA) so I don't know if that number is the same for your area, might have to call sprint and check!