It was a recommendation to At-Large to consider the free tier for Slack at the time (around 2017 or so (time flies!)) given the advantages of group chat over Skype. ICANN At-Large Slack was created back in 2014 and used in ATLAS II.
The problem now is that Slack in the last year has changed the free tier to only search the last 90 days of messages and only have full access to past messages if you pay. There is a tier for non profits up to 250 users with full functionality - our ALS (Trinidad and Tobago Computer Society) applied for this about 2-3 years ago and it has worked well for us.
I still believe in group chat but some analysis and tests will have to be done (Teams ? Signal? Jami ? )
Re: phone calls - this was an area where Skype shined - it was also free to call 800 numbers in the US so you could use Skype to dial the 800 number for Adigo to select the language channel ( since the Zoom audio channel was, until recently, was only broadcasting one Adigo channel in or in the case of f2f meetings, the uninterpreted audio)
A replacement for Skype to dial phone numbers ? Maybe Keepcalling?
Dev Anand