Secondary call disconnecting kills audio on established/primary call

Currently calls will autoreject secondary calls, because it does not implement call waiting yet.

Once the second call gets disconnected the audio for the existing call is broken (both from and to the network).

Note that the broken audio is not due to calls autorejecting the call: Even if the call get's disconnected from the caller audio gets broken but is still fine while ringing.

Some observations:

• Audio continues to works just fine on the PinePhone when the second
• I've disabled auto hangup in calls to test if it makes any difference who hangs up on the second call: Audio also dies if the caller hangs up.
• I tried using older MM (1.14.12-0.2pureos1) which did not use QMI voice call management to see if that makes any difference, but any additional call gets routed to voicemail irrespective of the call waiting status.
• Once the second call is disconnected the modem audio card does not produce any more audio samples. When running parecord on the modems audio card: The instant the second call is disconnected the recording stops (as opposed to continuing but recording silence).

As the modem audio simply stops it would point to an issue with the modem (firmware), but another opinion or ideas about how to further narrow it down would be welcome /cc @angus.ainslie @sebastian.krzyszkowiak @martin.kepplinger

Workaround

A possible workaround would be to mmcli -m any --voice-disable-call-waiting. In this case extra calls go straight to voicemail (or rejected as "busy") and I get notified of the missed call by SMS from my network operator.

We could disable call waiting entirely in Calls for the time being to prevent audio getting killed.