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.
Skype for Slack - Move your slack discussions to a group skype call
Tauria - True end-2-end encrypted group calls for up to 50 people
Aircall Now - Make voice calls inside Intercom Messenger 📞💬
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Slack - A messaging app for teams who see through the Earth!