Skype for Slack - Move your slack discussions to a group skype call
Daily.co for Intercom - Free video calls & screen shares embedded in Intercom
Calls on Slack - Make voice calls in Slack
Pluot Communications - Big-screen video conferencing for startups
Pingly - Your communication hub — Email, text & voice in one app
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.