How to take my service off my old iPhone?

I just got a new iPhone and i want to give my old one to my friend but the service on my old iPhone still has my number on it. Does anyone know how to make this situation go away? I'm not looking for airplane mode or turning off the cellular data, all i want is for my number not to be associated with that old iPhone.

If you have completed your 2 year contract with that phone, or paid full price for the phone, you just need to contact your provider and get it unlocked.