They work fine for data and SMS, but it gets complicated once you need audio routing (it's rare for a modem to expose audio over USB) or waking up from low power mode to answer the incoming call. Could be done with M.2 USB modules and some dedicated controller in-between though.

HID USB devices can already wake up computer from sleep, so i dont think we need M2 here.

Also i dont think routing audio is problem, the dongle could represent itself as external audio device, like those external usb dac.

Of course it could! Now find one that does.