What phone do you have??
The reason I ask is I had a Nokia which was really really rubbish in the car, could not hear caller and they always assumed I was talking to them whilst on the pot
Then my boss kindly got me an iPhone.... wow what a difference, on full volume I can hear every word clearly and callers have commented on the clarity of the call....... they mainly say "stop flooring it all, I can hear is your zorst"
Might be worth trying a different phone just to see?