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E-mail? (PGP encrypted and signed if needed)

WebRTC-based software? (e.g. Jitsi Meet, https://meet.jit.si)

My parents, who work in both China and the U.S., use telegram

Telegram maybe? Not sure if it works in China though.

Also blocked in China

We use Zoom - that's about the only thing left these days that works reliably other than a self hosted VPN tunnel.

Zoom works fine. Teams was working as of a week ago.

How about xmpp?