Calls on Slack - Make voice calls in Slack
Skype for Slack - Move your slack discussions to a group skype call
Rebel Calling - Internet-free international calling for $1 a month
Pingly - Your communication hub — Email, text & voice in one app
Smart Slack Reminders - Schedule follow-up questions to any message in Slack 💬
HandsFree 2 - Call and SMS in Mac using your Android phone via Bluetooth