I am on an Evergreen L5, with gnome-Calls 43.0, pulseaudio 14.2, with recent new L5 base packages installed.
Calls are working for me with VOLTE on and on 4G AT&T.
What i did notice is that if the Camera/Microphone HKS switch are set to off, then receiving or making calls on selecting answer it goes straight to voicemail. This could be by design.
I think what it should be doing is allow calls to go through, then with microphone off send banner notification to user alerting them that they need to turn the microphone on to be heard, maybe even have the action in the notification if the microphone is turned off in software instead of the HKS which would take the user to the Sound Gnome settings menue.
Not saying that would be a fix but maybe make the software more robust.