Calls on Slack - Make voice calls in Slack
Skype for Slack - Move your slack discussions to a group skype call
Slack - A messaging app for teams who see through the Earth!
Pingly - Your communication hub — Email, text & voice in one app
Talko for Slack - Team calls for mobile & web in any Slack channel
HandsFree 2 - Call and SMS in Mac using your Android phone via Bluetooth