Today our iPhone 4s with the same issue, was fixed. Our local dealer did what he called a SOFT SET He held the ON button at the bottom, and the button on TOP, down, simultaneously, and waited for the APPLE icon to appear. After that, the phone worked perfectly. We now can hear the caller's voice perfectly.
I had the same problem and I was reading on here and did the SOFT SET. Then I realized the plastic sheet was still on the brand new phone. WOW!!! it worked like a charm. Man, I can be a goof sometimes. It was a good laugh anyway. I had the whole office trying to find out what was wrong.