I am having same issue with a 2013 G37x prem/nav. No other phone was ever paired with the car. Dealer paired it. Outgoing calls are fine. If I hold down home button on phone, Siri comes through BT just fine. BT Audio mostly works (sometimes track info and track control don't work, but streams fine).
Problem is incoming calls. Phone rings through BT audio with actual ringtone. I hit Answer via steering wheel controls. Neither party can hear the other through BT, or through handset. If I use steering wheel control to transfer call to handset, can communicate fine through handset. If I then hit the Phone button on the steering wheel the call is transferred back to BT and it works fine. But very next call, same problem. To put some matters to rest, I borrowered some phones and another car and tested --
iPhone 4 - iOS5 - 2013 G37x - works perfectly
iPhone 4 - iOS6 - 2013 G37x - works perfectly
iPhone 5 - iOS6 - 2013 Acura TL - works perfectly
iPhone 5 - iOS6 - 2013 G37x - incoming calls connect, but no audio to either side.
I unpaired phone at both ends, and repaired - did not fix.
So problem is clearly not the phone operating system. It's an incompatibilty between the iPhone 5 hardware and whatever head unit Infiniti is using (or BT chip). I'm not sure who is at fault, but the iPhone 5 is newer. It should be backward compatible. My guess also is that plenty of other cars other than Infiniti/Nissan use the same headunit/BT chip and will have the same problem. I think Apple needs to fix this. I plan on calling Apple Support as well as Infiniti Support. -John