Why can't I hear any sound during phone calls only?

So, this morning, my phone worked fine. I could take and send phone calls and both parties could hear each other.

Now, I can't hear the person I'm calling can hear me, but I can't hear them. This is ONLY during the actual call. I can hear everything when I'm playing games, etc., and even the dial tone. I can receive calls, but I still can't hear anyone when I'm the one being called. Luckily my husband is a trooper and is fielding the calls from afar.

So far, this is regular phone calls and Facebook Messenger. Again, I can't hear them, but they can hear me.

What on earth is happening? This is my second Galaxy S4 with this same exact issue.

I've also noticed that my phone gets really hot when I'm using it.

I've tried:

-Rebooting phone

-Uninstalling the app I installed this morning

-Letting my phone cool off before trying more calls.

-Factory reset

-Taking out SD card

Help please!!! I use this phone for work.