I guess that some parts of iPhone hardware are made by more then just one supplier, and are produced in batches that may vary a bit (or have a hidden fault or irregularity that appears under certain conditions, like a combination od carrier network configuration, sim card type - sim card is a chip, too - etc.), and that just some portion of iPhones is affected then.
Solution: do not wait for a software fix, get the faulty phone replaced.
Today, I called the customer line of the online Apple store in my country I got the phone from. They immediatelly knew what I'm talking about, but had to switch me to their tech support first - however as I had already call the tech support a week ago and we already went through the list of checks (switch on/off, factory reset etc.) and they had indeed this record - they said immediatelly I'm elligible for a replacement with no further checks.
The customer care person I was talking to was really apologetic for the situation and nobody questioned this is their fault.
When I was talking with my carrier support a week ago, their advise was the same: if you brought it in our store, come back to get a replacement.
I'm a bit disapointed, indeed, but on the other hand, with such a complex systems, this must happen from time to time.