So why then does Slack launch Skype if I click on a number?
Loads of people here suggesting switch from Skype to Slack.
Well, my installed Windows version of Slack converts numbers in posts to hotlinks. Clicking on one always launches the Skype login prompt (Skype being installed for other reasons like doing remote desktop shares - Lync replacement). Slack thinks it needs a phone app to dial a number.
So installed Slack has tight integration with Skype and I have yet to figure out how to remove it.