mic is muted all the time, icon greyed out, unable to unmute

[AC Question]

My Nexus5 has is running android 6.0.1 and has the default phone app that came with the phone. The other day I got a call where the other party could not hear me. I noticed the mic was muted and the icon was greyed out. If I tap the icon it's a highlighted muted mic. If I tap again it's a greyed out muted mic. I check the phone app's permissions and it has permission to use the mic. I rebooted the phone several times with no fix. I downloaded a new phone app and it too has the mic pertinently muted. If I switch to speaker phone or use a headset people can hear me.

Any ideas why I can't unmute my phone?