Joined the Mayqueeze.
I fear there won’t be a solution that doesn’t cost money somewhere. Please don’t buy 3 burner phone numbers at once. I would feel bad if it didn’t work and/or you get suspected of being a criminal.
Check your current phone subscription if you have one. There might be a way to get an additional number for free or very little extra. And it might be a good backup emergency service to have just with your parents maybe even if you go down a different route for the rest.
Anything other than Signal seems unadvisable from a privacy POV. Do you have a way of talking to them live on the phone, at the same time? You can try and register their Signal on a burner phone wherever you are. As they sign up, you sit on the call and give them the phone number and security code when it comes through. Unless having Signal on the phone is itself a reason to get arrested in Russia, which frankly wouldn’t surprise me either.
I don’t know if this will work. I did it on a different service using the number on a dumbphone to register for use on a tablet once.
They are a bit vague on this but I suspect all of these attack vectors start with LEOs having physical access to the unlocked phone. They then set up a trusted desktop without the phone owners knowing.
Which is clever, to be fair. Whether or not that’s legal is already a court case. The law is so frightfully grey.