How do I activate my iphone 4 phone number? My old iPhone broke and want to activate this new one with the same number

Long story: My son broke his iphone 4 screen through repeated droppings. But my wife upgraded from her iphone 4 to the 5, and wants to give said son her unblemished 4. He has saved to icloud and we have deleted all the content and restored settings on wife's iphone 4. So on the unbroken iphone 4 all of the son's contents, contacts, app, etc., are there, but how do I activate the phone with his number? so he can receive calls/ texts?