Free and Open Source Software

Hi all! I'd like to request suggestions for a secure messaging app, ideally that doesn't require mobile phone for registration, to stay connected to my family In Russia.

The country wages war on the Internet, messengers and VPNs. Options are blocked one by one, and one can't register in Signal because numbers that send registration confirmation from Signal are blocked...

I'd need an app that allows group chats, calls, media attachments and audio messages, easy enough for older people would be able to install. Ideally, something niche enough it won't be blocked right away...

It's a lot of requirements, but I hope something like this exists and would be very grateful for any recommendations.

Android / iPhone / desktop.