Hi Pirmin,
thank you for testing the PinePhone image.
After some more testing I was able to turn on and off the modem multiple times. So I'm not sure, why this wasn't working on my first try.
Unfortunately I still wasn't able to do calls.
The modem_pinephone.run script at the Allwinner repository [1] might be helpful to solve this mystery. It allows you to interactively talk to the modem using AT commands. You can find the commands for placing and receiving calls documented at the beginning of the run script. More detailed information is available at [2] in Section "LTE module information".
I wonder, are you able to unlock your SIM and issue/receive voice calls using this manual procedure? If yes, I would very much appreciate a log of the AT commands that work for you. So we can extend the AT-protocol driver [3] accordingly.
[1] https://github.com/genodelabs/genode-allwinner/blob/master/run/modem_pinepho... [2] https://wiki.pine64.org/index.php/PinePhone#Datasheets_for_components_and_pe... [3] https://github.com/genodelabs/genode-allwinner/tree/master/src/drivers/modem
Cheers Norman