Pinephone woon't stay registered

[jhasler]

Installed Biktorj's modem firmware.

Made a difference: the modem now connects and stays connected for at least an hour. However, I still cannot complete calls. My first try went through but was terminated (reason "unknown") after a few seconds. Subsequent attempts get

couldn't start call: Couldn't create call: QMI protocol error (90): 'IncompatibleState'

followed by

[modem14/call3] call state changed: dialing -> terminated (unknown)

Incoming calls get sent to voicemail. Now, after an attempt to call the phone, the log is filling up with

<warn> [modem14/bearer20] reloading stats failed: Couldn't get packet statistics: QMI protocol error (15): 'OutOfCall'

On a second attempt to call in the phone rang but the ringback continued and the call did not complete. Log:

<info> [modem14/call4] call state changed: unknown -> ringing-in (incoming-new)
<info> [modem14/call4] user request to accept call
<info> [modem14/call4] call is accepted
<info> [modem14/call4] call state changed: ringing-in -> active (accepted)
<info> [modem14/call4] call state changed: active -> terminated (unknown)
<warn> [modem14/bearer20] reloading stats failed: Couldn't get packet statistics: QMI protocol error (15): 'OutOfCall'
<info> [modem14] 3GPP registration state changed (home -> searching)
<warn> [modem14] couldn't load operator code: Current operator MCC/MNC is still unknown
<warn> [modem14] couldn't load operator name: Current operator id is still unknown
<warn> [modem14] couldn't load operator code: Current operator MCC/MNC is still unknown
<warn> [modem14] couldn't load operator name: Current operator id is still unknown

and then back to:

<warn> [modem14/bearer20] reloading stats failed: Couldn't get packet statistics: QMI protocol error (15): 'OutOfCall'

Seems like the modem is working but modemmanager isn't.