@prosodyim is free open software for creating your own XMPP/Jabber server, a federated messaging and calling service. Because it's federated you can message and call people on other XMPP/Jabber servers.
🪴 If you're moderately techy you can install Prosody via @yunohost at https://apps.yunohost.org/app/prosody
🌳 If you're very techy, you can install Prosody manually using the instructions at https://prosody.im/doc