This means no sim or the modem goes crazy pinging about for a tower in a Faraday sack and burns off battery% though I guess it still pings away for emergency services calls. There is a non-root command to ignore modem etc in the phone and not have the android calling app show,
setprop gsm.radio.enabled true(or false)
then I recall it needing a reboot for the change to take hold. I am not sure exactly how much that truly affects modem though since the modem is super tightly tied into android hardware to the point that it shares RAM DMA.
I posted a few weeks ago how to BT tether from your Librem, that saves some battery on both devices. Bluetooth PAN network tethering - #4 by biketool