Skype for Slack - Move your slack discussions to a group skype call
Arvia for Slack - Start quick video calls in Slack channels.
Calls on Slack - Make voice calls in Slack
Moon Dialer - Use free voip & wifi calling app for unlimited international calling. Moon Dialer is one stop solution for call recording, call scheduling and call forwarding.
Pingly - Your communication hub — Email, text & voice in one app
Viber - Free and secure calls and messages to anyone, anywhere. Keep your conversations going no matter where you are.