Calls come to Slack — Several People Are Typing — The Official Slack Blog

Audio + emoji

"Anyone can make one-on-one calls, while paid teams can also make group calls in any channel or group DM. When someone initiates a call, a message will appear, inviting you to join the call.
Best of all, calling your teammates in Slack saves everyone the fuss of digging through calendar invites for call numbers and dial-in codes and all the rest. That’s a bit of worky-work we can probably all let go.

Calls even have an emoji response feature. Clicking one of the pre-selected emoji will display it over your avatar for a moment along with a subtle sound, allowing you to express approval, disapproval, or raise your hand to ask a question — all without interrupting the speaker."

Calls come to Slack — Several People Are Typing — The Official Slack Blog