Librem 5: It should be easier to enable/disable VoLTE

Some carriers, e.g. in the US, are moving to VoLTE-only support, and booting devices off the network that don't support it. Good news is that Purism has done some work with US carriers to fix-and/or-enable VoLTE for the L5 modem. We just need to figure out how to integrate the necessary config/commands into pmOS!

Once enabled or disabled, the config is persistent across reboots. So it shouldn't need to be re-applied.

Here are some options for how this could be implemented:

1. Purism has a GUI app that can toggle it, bm818-tools. I think it's GTK, so might not be great for plamo, but it's probably better than nothing.

• I think this might be the best option, tbh, since it also neatly shows current status of VoLTE. Purism may ship phones with it enabled by default, so supporting this would allow users to easily see if it's enabled and disable it if they want.

2. Users can install a package that enables VoLTE automatically, or uninstall it to disable VoLTE

   • create a script/subpackage of the librem5 device package
   • set to run on first boot (for pmOS images) or from a post-install trigger (for current installs)
   • uninstalling it runs some pre-deinstall trigger to run script again to disable VoLTE
   • not UI-specific, but has no GUI. This means that 'toggle' VoLTE via the package manager, basically.

3. Something else?

For reference, here are my notes on enabling and disabling VoLTE:

To enable it:

sudo screen /dev/ttyUSB3
  ATE
  AT+CGDCONT=2,"IPV4V6","ims"
  AT$QCPDPIMSCFGE=2,1,0,0
  ATE0

# power cycle modem with kill switch or reboot

To disable it:

sudo screen /dev/ttyUSB3
  ATE
  AT$QCPDPIMSCFGE=2,0,0,0
  ATE0

# power cycle modem with kill switch or reboot

If doing option 2, the "sudo screen" should be replaced with something more appropriate. Also keep in mind that users might not see any message to reboot or toggle modem power after they install the package, e.g. if they use apk indirectly through some GUI. So maybe using a dedicated UI tool (option 1) is better? Or we provide both options (with more work)?

@alistair23 do you use a carrier who supports VoLTE? would you have any interest in taking a look at this?