Intermittent loss of voice on calls

I’m trying to figure out why my iPhone 6 intermittently loses voice on calls. Sometimes it works fine, sometimes I can hear the other person, but they can’t hear me, and sometimes there is no sound on either end of the call (this is the usual scenario). This problem seems to be getting progressively worse. My online research pointed to a possible software issue - I started with rebooting phone, then reset network, all the way to restoring phone with no improvement. The phone still plays music & records memos fine. When the phone is connected on a call and does not have sound, the icon to tap to turn on speakerphone is grayed out. At this point, I’m thinking it must be a failing component?? I’m just not sure which part I would need to replace. Thanks for any input.