APPLE PLEASE SHOW US SOME COURTESY AND ACKNOWLEDGE THIS PROBLEM PUBLICLY.
I have exactly the same problem with my 32gb iPhone 4S. Intermittent OCNA worse after using headphones and often happening in clusters in general one out of four calls ever since I bought the phone in October. Almost always resolves after putting into speaker phone mode and back out but recurs soon after.
I called apple telephone support and they pretended they had never heard of such a fault. Told me to erase and restore my phone.... Several hours later the problem was still there. Called them back and they decided it must be a hardware fault so made me an appointment with a "genius" at an apple store for a phone replacement.
Apple store "genius" had also apparently not heard of the problem. Checked the diagnostics on my phone to no avail. Agreed that if erase hadn't worked then it must be hardware. Swapped my phone for a new one and restored from iCloud.... However the problem returned.
Then erased phone and set up as new phone- seeded to work. They concluded it was a problem with my backup, probably as it came from an old 3GS. Their advice: start afresh with a new setup but no way to restore text messages or app data. I'm not keen to do this as I'd miss all my text messages. This is very much a work around rather than a fix. I hope it is addressed in IOS 5.1
Whether it is or not I think I would like to reiterate the sentiments of many of the previous 2380 comments and the thoughts of many of the 273842 views of this discussion by saying, as above:
APPLE PLEASE SHOW US SOME COURTESY AND ACKNOWLEDGE THIS PROBLEM PUBLICLY.
It would be nice to know that it is being addressed and that you care about your customers problems with your otherwise good but very expensive product.
Thank you.