Hm. I'm with thomasjtsi. If you've checked your settings and performed a hard reset, then I'd suggest doing a factory restore through iTunes and setting up the phone as a new device.
If that doesn't help, then you know it's a hardware issue and luckily your still in your warranty. Apple will replace your device. It'll just make the process quicker if you tell them that you've tried everything from checking your settings to setting up your iPhone as a new device.