There's some success with using VoLTE for phone calls (e.g. https://source.puri.sm/Librem5/OS-issues/-/issues/162)
I wonder if there was already some discussion how this should be handled on the MM side API wise (enabling/disabling VoLTE and indicating the current status)?
The AT commands used above changes a PDP profile so this looks rather device specific. Can MM be bothered at all?