Not saying there's a turnkey solution at the moment for your exact needs, but even with your current $5k/year budget something interesting could be built with the "modern IRC stack" that
https://irctoday.com/ uses, for example. Call support would have to be done by integrating Jitsi, but it's been done before as seen in
https://github.com/kiwiirc/plugin-conference and
https://convos.chat/doc/features#video-support