VoLTE support

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?

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