XMPP is lightweight enough to run on the tiniest device. But supports messages and voice/video calls.

It is annoying with multiple devices on the same XMPP address - logs are split/duplicated between devices, and it is unclear which device will notify you of a new message or call.

I was on twitter through XMPP until they cut off anyone without a twitter account. Facebook was the same thing. They talked a protocol driven game ("like email"), then cut me off.

Matrix is really good at the message store on server - but we're talking 2G+ ram VM for the server and 40g+ disk.