Iphone xs sound during calling is bad

If you mean the earpiece speaker is muffles that can be due to either dirt or the speaer has been damaged.

If it is the latter you can get by with a change to the accessibility settings.

Settings, Accessibility, Touch, Call audio Routing, Speaker and use the bottom speaker all the time.

I had to do that when I dropped my iphone 12 and got the same symptom.

PS Need to think in the region of $400 for a repair

Third party repairers will wreck the face ID and portrait mode if you have it.