How can I unlock a used iPhone?

Jake bought a new phone on eBay, but he can't activate it because it says he owes money to AT&T — but he's never used it with AT&T. Leo says it's probably the carrier that has flagged the IMEI of that phone and locked it out. Leo advises contacting eBay and getting his money back. He should give the seller a bad review as well. Of course, he could pay the bill. But who wants to do that? This is why you don't buy phones on eBay.