Might just be the Sprint network. I use the iPhone 11 on Sprint, previously a XS Max. Both have had VoLTE enabled. I noticed that if a particular area is experiencing degraded data performance the call will be choppy on LTE. I have run into that a few times, once while visiting NYC in April. Had to turn VoLTE off while in lower Manhattan. The others I live in the Chicago area and VoLTE can become choppy during a crowded event or extremely low signal.
The temporary fix: revert back to “LTE, VoLTE off” under cellular data until the tower goes back to normal operating conditions.