Consult with an attorney, one with experience in identify theft and fraud cases, before doing anything. The main issue is actually not whether or not you were able to pay--the primary issue is that you committed both identity theft and fraud by using your son's social security number to secure the phone service and phone. If you come forward voluntarily and also agree to pay the appropriate restitution, there is a good chance you will only suffer a minimal punishment and may be able to avoid jail time entirely--but given that the possible consequences are serious, you need legal representation and consultation before acting.