My experience
I posted this on another thread, but maybe it belongs here.
I'll try to be brief: took ip4 into Apple store today. The genius set up my iphone as new, blaming faulty data since I had backed up from restore. Left store, made one call. It failed. Tried again, sounded like I was underwater. Tried again, reached my friend, call dropped. Tried again, she said it sounded as though I was underwater. Tried again, call dropped.
Went immediately back to Apple store and convinced them to give me a new phone. The guy there restored it from the backup (in spite of my protests.)
Tried to make calls; totally still messed up.
Called and got really great apple support person. He tested my phone by calling - his call went straight to voicemail. We set the phone up as new. No data on phone. Tested. Call failed. Removed and replaced SIM card, tested, all calls went straight to voicemail. He believed me. He said he will report this. He said Apple is aware of problem and is working on it. I am going to ATT corporate tomorrow to get a new SIM card to see if this could be it, and I will call him back (he gave me his direct number.)
He suggested going to apple.com/feedback (something like this) and reporting stuff that's not working - since real people supposedly read those things.