Galaxy S4 Rings Fine, I Answer, Phone Volume Goes To Vibrate By Itself

[Tigrisan]

I'm running 5.0.1, but this has been going on long before the update came through. The title pretty much explains what's happening.

Ring, ring...

screen lights up, I swipe the green phone to answer

yada, yada, yada,

See ya!

I end call with red phone icon.

Look at the phone and the volume has turned itself to vibrate.

I am NOT touching any volume buttons or anything else other than the answer and hang up icons on the touchscreen. This happens almost every time. I've done a reset, doesn't change things. I turned off the Do Not Disturb function, I installed Disable Increasing Ringtone app in the hopes that it would help. Nothing. Almost every single time the phone rings, once answered, the volume shuts itself down to vibrate.

Does anyone have any ideas? I'm at a loss. And please don't tell me to get a new phone. The ringer problem is the only problem I have with it and I'm not ready to replace it if I can help it.

Thanks!