I'm not aware of any non-Apple facetime clients. Are your friends and family all locked in to Apple just because nobody wants to try a different chat app? I know plenty of people who don't have any facetime client whatsoever because they don't have any Apple products whatsoever. I suggest that "nobody else you know" wants to migrate to new chat software because nobody's asking them to.
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?