Just in case anyone else is experiencing this problem. If you find the sound of the other person cuts out intermittently during a call, or goes very low, it may be the result of the slider on the N80.
Originally I wasn't sure whether it was a network or software problem, but have now discovered the root of the problem. When I have the slider up during a call, if it moves from side to side a little, it will cut out the sound from the other person or make it go very low.
Considering how many posts there are in this forum regarding issues with a loose slider, (I never really considered mine to be loose) I shouldn't be surprised if others are experiencing a similar problem. My phone is now with a nokia service center being repaired, but I would be interested to hear if anyone else had this problem.