Matrix. With its bridges you can “wire-in” networks like WhatsApp, Slack, Signal, Telegram, Discord, iMessage, SMS, e-mail, … and have a single app that interacts with them all. You can have a single group chat with users from all those networks participating and no one would be any the wiser.

If you’re already using Nextcloud, it has a chat w/ video chat as well.

Matrix / Synapse / Element.io is also pretty cool. The UX might not be on par with what some family expects though. I don’t know if voice/video chat is built-in yet or not, but it was at least an option before.

Matrix works, but it’s way harder and more expensive to selfhost than for example XMPP, which can be hosted even on cheapest VPS or first RPi. I would definitely take the cost and “how hard is it to maintain in the long run” into consideration.

Mattermost also works and is pretty easy to selfhost, but it doesn’t have federation.

Another option is always an email with delta.chat - I don’t think it offers voice calling, but email is one of the most basic services one can host, and many automated solutions to help with that exist.