IPHONE Hardware Repair

Hey all Apple have said they will collect my phone for a hardware repair as it has a strange fault that occurs only now and then with the touch screen. If they find that it is OS firmware related will they charge? or do they just tend to receive the unit back then swap out the phone?

Thanks

If they find that the problem is due to user tinkering with the OS or firmware, they will either charge or just send it back unrepaired, since that violates the warranty.