Call Quality is Horrible

It's been trash for me also. I've actually had this problem starting with the s7 edge. My note 8 and my s9+ all the same. I keep thinking its tmobile but I'm not sure. Getting to really think its Samsung because it's the same with my wife's note 8 and she didn't have this problem with her LG G7
 
Both my S9+ and iPHone X sound good on AT&T. Even my Google Pixel 2 XL. It can also be the quality of service in your area. Did you try to see if there was a difference when using WiFi calling? If calls sound better over WiFi then it can definitely be the carrier service in your area.