I use deltachat/arcanechat. It uses the mailchat protocol. There's also tox, simplex, matrix, xmpp(conversations) these can do e2e, you can self host it or use a public server, everything distributed through app stores can be compromised. Every app were you rely on a central server can be shutdown. You also don't know how the server software works of for example "signal", what data they collect. Although messages are encrypted you can still log metadata like when where iplocation, with who?

The problem i guess with thr alternatives is bad service in terms of audio and video chat. The benefit of ms teams for example is their huge cdn. it just works nearly all over the world. Whatsapp/discord is used by so many that it's difficult to stay in contact otherwise.