Skype for Slack - Move your slack discussions to a group skype call
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Calls on Slack - Make voice calls in Slack
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Pingly - Your communication hub — Email, text & voice in one app
Better Uptime - We call you when your website goes down