Skype for Slack - Move your slack discussions to a group skype call
Timy - Best way to send scheduled messages in Slack
Calls on Slack - Make voice calls in Slack
SKEDit Scheduling App - SKEDit Scheduling App is a rich-featured scheduling application that entitles you to automate your messages by scheduling your WhatsApp messages, SMS, e-mails, post, and set call reminders to experience a healthy or tension-free lifestyle.
Pingly - Your communication hub — Email, text & voice in one app
Slater by Convergely - Schedule your Slack messages now, post them later