Is there a way to somehow "reboot" or restart the layer1 by software? And if not, could that easily be implemented?
Yes there is the L1CTL_RESET but I think shutdown/no shutdown already sends that command (check the osmocon output) meaning the L1 is no longer responding at all ...
Does the osmocon console does anything at all when it's in that state ? Does that phone have anything special ? (connected to a different network or doing different things ?).
Some people wire-up one of the serial port control line to the enable line of a power supply they use to power the phone so they can do full power off/on, but it requires external hw.
Cheers,
Sylvain