I haven’t had any of the audio problems you describe. I live in NYC metro area so maybe the signal is better here than in other parts of the country. But even when I go to Boston or DC, I get a strong signal.
I’m out in flyover country...weak signals are a fact of life..in many areas.
It seems to me that the audio issues were related to the Iphone 5 and 5s....I have not had the opportunity to observe the 6, but my cousin has one and the last time she called me I could hear her clearly for the first time in a long time...lol