Iphone 5 was stolen

My Iphone 5 was stolen yesterday. I try trace my iphone using "Find Iphone" Apps using my sister iphone. When I use Lost Mode & leave my contact number, the person call me back. But he claimed it his iphone. He said his iphone is Iphone 4S. So how did "Find Iphone" trace the iphone? using the serial number or only by apple id?