Calls on Slack - Make voice calls in Slack
OpenPhone - Simplest way to own a business phone number
Skype for Slack - Move your slack discussions to a group skype call
Yodel - Group photo and video sharing. Create stories with friends, join stories from around the world.
Talko for Slack - Team calls for mobile & web in any Slack channel
Houseparty - Brining empathy to online communication.