Skype for Slack - Move your slack discussions to a group skype call
Smart Slack Reminders - Schedule follow-up questions to any message in Slack 💬
Calls on Slack - Make voice calls in Slack
RemindUs - Advanced reminders for Slack teams
Pingly - Your communication hub — Email, text & voice in one app
PullReminders - Review and release pull requests faster with Slack reminders and metrics.