My ex keeps prank calling me from different numbers?

Dont answer the phone. See him in person and tell him he's pathetic. If he's your ex its obviously for a reason and he should leave you alone to get on with moving on. If it carries on id say go talk to someone about the fact your being harassed. Its not good for you. Unfortunately there's no 'tracer' you'll just have to catch him yourself, maybe get a friend to be there while he makes the call? record his voice? or catch him in the act? CHIN UP.