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
Calls on Slack - Make voice calls in Slack
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Pingly - Your communication hub — Email, text & voice in one app