Calls on Slack - Make voice calls in Slack
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Rebel Calling - Internet-free international calling for $1 a month
Mitel - Business Communications and Collaboration
Skype for Slack - Move your slack discussions to a group skype call
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.