How to unlock an iphone4 without knowing what network the phone is locked to?

So was was walking home from school the other day and luckily I found an iphone on the grass I was very happy but the oment I tried my sim "3" my happiness shattered because the phone was locked to some network it shos "no service" at the top and when I connect it to iTunes it shows that he sim is not "valid carrier".so I want to get this phone unlocked but I don't know what it is locked to a it doesn't flash the company logo when I switch on the phone… Please Help

Return the phone to its rightful owner

Be a better person and return to the rightful owner. What goes around comes around

Apple would be able to tell you what carrier your stolen phone is under. But they can't unlock it.

If your very lucky, the 'Find my iPhone' app has been installed and has taken pictures of you trying to use the stolen iPhone. The police will be able to use these when your case come to court.