I'm the sole it guy for a small university in Germany (~40 FTE users and ~200 students) and here is my take on these tools. I used or tried to use most of them and have a, what I think is, good take on what I can burden my users with. I'm also bored this Friday evening because I can't go anywhere, so...

Mumble: Mumble is amazing. I used it extensively playing EVE Online and what can be done with mumble with regards to external authentication and integration is really great. As an enterprise solution, if I have to weigh my time versus expenditure, it's not worth it though, I'll go with MS Teams or Slack.

Asterisk/SIP: I actually ran an asterisk PBX with a self made IVR solution for a project at my previous employer and handled a lot of calls with it and saved a load of money compared to what the callcenter would have charged. Asterisk is great but we are back to the time/expenditure calculation again. I can buy a PBX solution and pay someone to maintain it for far less then me setting up Asterisk. I'd rather just buy a PBX and a support contract. For IVR the calculation gets more complicated.

Jitsi: Jitsi meet is really great. It's right in that sweetspot where Skype begins to fail for decent conferences (>4 people) and where I'm invested that much that I want someone to blame and a service contract if my meetings get large enough. If you exceed that threshold (in my testing it has been somewhere between 15 and 30 participants) you want to pay someone for this service or run your own server/videobridge (still curious why BigBlueButton isn't mentioned here anywhere in all these Zoom topics). As an educational institution with my amount of users, hosting a videobridge or bbb the hosting budget alone would have exceeded our Zoom licensing cost not to mention my time. At least in Germany there is also the issue with not having a contract with the provider and problems this creates with regard to the GDPR.

Jami: I don't actually know this one.

OBS: Nothing can touch OBS. It's an incredibly flexible tool and while it could use some usability improvements it just really is that great. With it's usability downfalls it's also not something you can suggest to a casual user, which is kinda sad. Someone please figure out whatever Zoom is doing with the virtual camera background. Setting a simple picture as background without a green screen frankly is one of their killer features. No one talks bout it but so many people are using this that it has got to be one of their killer features.

XMPP: Is a weird mention for me, since it's not an application but a protocol. Jabber sucks, most XMPP clients suck for normal users. I'm not aware of any decent enterprise solutions.

IRC: Well, IRC is fucking great. I spend so much time on IRC in my youth and I'm fairly certain my earliest linux contact was an IRC bouncer on a friends machine. In the end it's kind of a weird mention as well though, since it's a protocol and not an application. It is also a protocol that is kinda caught between the "I want link preview, VoIP, file sharing, whatever" crowd and the "why is that noobs client spamming my channel with un-interpretable messages" crowd. IRC with a spec upgrade, even if it would induce a permanent netsplit would be great.

I'm not gonna talk about the rest. Nextcloud is great, and with Univentions app server (https://www.univention.de/) is a usable solution that just suffers at scale. Of course I could set up a univention server and nextcloud and imap email and ldap and lots of other things mentioned above but it would cost me a significant amount of money and time and I can have Office 365 basically for free. So, considering I'm spending tax payers money, and I'm in the EU and MS claims to be GDPR compliant... what is my incentive to do any of this?

If I'd have one complaint about Zoom, I'd would be that we had one student unable to join from Iran (which is either because of his ISP blocking certain connections or Zoom actively blocking users from Iran). Zoom excludes users from Iran in their ToU but we assumed that would just be for paying users and have a currently open ticket to figure out what could be the issue.

So, this is my take. Basically if you have the time and money and want to tinker most of these OSS solutions are great and worth it. If you are operating within budget or time constraints there a valid reasons to choose a commercial provider.