You can now make Skype calls within Slack.
Microsoft released on January 14 a preview version of Skype integration for Slack. The preview enables teams on Slack to make Skype voice or video calls from within Slack. Slack is an instant messaging-collaboration platform for teams. Anyone on the Slack team will be able to start a Skype call from a computer by typing “/skype” into the Slack chat regardless of whether team members are using Slack on the Web, Windows, Mac, iOS or Android, according to a Microsoft blog post announcing the integration.
From ZDNet.