But those apps don't actually care about the phone number beyond the initial registration? Once registered, they work entirely via IP and don't know nor care which SIM is in your phone.

You will however have a problem with registering on those with a VoIP number - those ranges are generally blacklisted due to bad actors misusing those numbers for nefarious purposes.

Could use a VNC of sorts. Maybe a Miracast receiver that dumps the screen buffer to your own web service that just spits out the frames over UDP