Skype for Slack - Move your slack discussions to a group skype call
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Aircall Now - Make voice calls inside Intercom Messenger 📞💬
Mitel - Business Communications and Collaboration
Calls on Slack - Make voice calls in Slack
VoIP.ms - VoIP.ms is a VoIP origination and termination tool.