Enhanced Call Audio symbol.
It basically means you boosted the bass - vs flat - vs treble high.
(it may be why everyone sounds muffled)
To change it, just go to your phone screen > open menu > options > general options > enhance call audio.
Change the settings to what you like. Generally, I keep my at Boost Treble. I'm on a tiny speaker - why do I need boosted bass response? Not like it's going to do much anyway.