Rebel Calling - Internet-free international calling for $1 a month
Skype for Slack - Move your slack discussions to a group skype call
Aircall Now - Make voice calls inside Intercom Messenger 📞💬
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
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.