Nextcloud, Matrix, Discord, Signal, Telegram, Skype, Zoom, hell even Pidgin. These are all competitors to Apple's Facetime -- they all have voice and/or video chat on Linux, Windows, Mac, Android, iOS.
Here, let me help you start the conversation that way:
> You: Hey mom/dad/friend/acquaintance/boss, I'm going to use a different program to talk to people who don't have an iPhone. Can you install it and talk to me on it for a bit and test it?
> Mom/dad/friend/acquaintance/boss: Yeah sure what's the app?
> Another mom/dad/friend/acquaintance/boss: What? No.
I wonder which way the conversation will go?
Perhaps they should be allowed to buy an appliance where most of their basic computing needs have great, widely used, secure solutions out of the box.