Slack is officially moving away from being a messaging service to bringing in voice calls. All users, whether you use Windows or Mac apps, Google Chrome extension, or Android and iOS apps, have access to one-on-one voice calls. You should see a phone icon in the upper right portion of the channel header for those on the desktop, while those using the mobile app can “Start a call” in the app’s channel or DM drop-down menu.
If you want to make group calls in any channel or group DM, you need to be part of a paid team. If someone from your team starts a group call, you’ll get a message prompting you to join the call. As mentioned before, calls also have the emoji response feature, which allows you to share what you think about the call or the topic you’re talking about.
Source: Slack