Re: So why then does Slack launch Skype if I click on a number?
"So installed Slack has tight integration with Skype and I have yet to figure out how to remove it."
No, it doesn't. It uses Windows associations to launch your assigned phone software. Numbers in my Slack launch 3CX for phone calls because that's what I have assigned. You can change the association or you can remove Skype. Either will fix your problem.