Sounds like you may need to look at environmental issues or software issues coming from your backup. It is highly unlikely that 3 Iphone 6 pluses would all have the same issue.
Test the mic with Voice Memos.
If you can hear the voice than the mic is fine.
Im leaning towards crashing apps that pertain to the phone app.
You need to restore your device and setup as a new phone.
This will remove all data from the phone.
You can bring in your contacts by logging into your Apple ID during setup.
DO NOT restore from a backup because clearly your backup has an issue.
It will continue to come back if you do.