There’s HipChat Server. However, since Atlassian launched Stride, I fear HipChat will fall behind.<p>My company does not want to use yet another free Slack alternative on GitHub. We’d love to pay and have a phone number to call if things go south.<p>End-to-end-encryption (as in: DB admin cannot read anything) would be a plus, of course.
Were using Zulip. Before that we used Mattermost but we migrated to Zulip because of its (in our opinion) superior conversation threading model and never looked back.<p>Mattermost, Slack, etc. all work fine when you work on a project basis but when you work on a lot of short lived operational issues (for example "Issue with invoice XYZ" Slacks conversational threading model breaks down quite fast. At least it did for us.<p>iirc there is also a paid hosted version of Zulip but I don't know that for sure.
Mattermost, works quite good. It is also pretty simple to set up and deploy.<p>[0]: <a href="https://github.com/mattermost/mattermost-server" rel="nofollow">https://github.com/mattermost/mattermost-server</a><p>EDIT: just read your post entireley.<p>>> 'My company does not want to use yet another free Slack alternative on GitHub'<p>So that one is not what you are looking for. But I keep the post in case anyone is interested in it...