This issue needs to be escalated to a firmware fix, getting reports coming thick and fast from customers like this one:
"I’ve noticed in the past few days while I’ve been on leave that people I’ve been talking to on the phone (through the normal phone mic) have complained that they haven’t been able to hear me clearly. They said speech was very faint to the point of being inaudible. I was able to hear them clearly.On one call in an effort to discover what the problem was I pressed the speaker icon and then they said they could hear me so I’m just wondering if there could be any known issue with the phone?"
One of the basic tasks of a phone is to make clear phone calls. We need an urgent fix.