I prefer Discord or IRC, but my problems with Telegram that I could see a business having issue with is how it's tied to a mobile device to setup (unless that's changed). That can get messy if you don't issue mobile phones at work.
Slack had the advantage of marketing, but I've never found much value using it at work over anything else. Slack is just another route to the "presenteeism" problem.
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I prefer Discord or IRC, but my problems with Telegram that I could see a business having issue with is how it's tied to a mobile device to setup (unless that's changed). That can get messy if you don't issue mobile phones at work.
Slack had the advantage of marketing, but I've never found much value using it at work over anything else. Slack is just another route to the "presenteeism" problem.