I posted about this last week.
https://forums.macrumors.com/showthread.php?p=19918661#post19918661
If you are mobile while on a call you have a good chance of dropping the call if you move between LTE and 3G coverage areas. If stationary and you start the call in LTE coverage area you'll get crystal clear fidelity.
Note this applies to BOTH participants on the call. Which means if you initiate a call to someone and they also have a VoLTE enabled phone then you both handshake into VoLTE mode until/unless one of you goes into 3G signal range and then the call will drop. But if only one of you has a VoLTE enabled phone then the call will handshake as normal CDMA and the dropped call effect shouldn't apply.
In other words....yes consider it a Beta with some "drop prevention" kicking in at call initiation to weed out drops except for total VoLTE to VoLTE connections.
Will say when it works....sounds fantastic. Except over BT headset as most don't support it yet.