I've been looking into telegram alternatives for a while. The best one I've found is the Matrix protocol (https://matrix.org), which is sort of like a cross between telegram and slack but it's open-source and you can self-host it. There are several app/webapp clients that you can use with your server, like https://riot.im.
It's got end-to-end encryption by default (although you need to manually enable it in some clients like Riot), Riot's mobile app works well, and supports audio and video calling through your server.
It's got end-to-end encryption by default (although you need to manually enable it in some clients like Riot), Riot's mobile app works well, and supports audio and video calling through your server.