That is set by the network. This is not the "enhanced 4g voice" toggle im talking about. This is the network set provisioning which lets the phone know if it can use VoLTE of not.
in the info menu, I can slide it to off - but exit the menu and come back in and its on "on" again.
It was like that on my Pixel XL. So I switched to a Pixel2 XL and it was not "on". it stayed off. Untill I went overseas and came home. Now its the same as the old phone - toggled to "on" and will always revert to "on" no matter how much it is toggled to off.
My port worked today. Since then *#*#info#*#* shows "volte provisioned" as off, the problem is gone and the new network is perfect. The new network also does not support volte, just like virgin.
Seems you have overlooked that many modern day android phones will automatically activate VoLTE if the network says its provisioned. So whatever flag your network is sending to tell phones they can use volte is causing the problem