Calls on Slack - Make voice calls in Slack
Skype for Slack - Move your slack discussions to a group skype call
Talko for Slack - Team calls for mobile & web in any Slack channel
Pingly - Your communication hub — Email, text & voice in one app
Freshcaller - Freshcaller is a modern-day reimagining of our everyday phone system for customer support, sales, IT, and HR teams.
HandsFree 2 - Call and SMS in Mac using your Android phone via Bluetooth