How do I phone call someone over Slack?

Go into a chat history you have with that person and click on their name above one of the things they have said to you. You will see a modal with an option for "Call" to click on. If you've never had a chat with the party in question, click on the plus symbol at the right of DIRECT MESSAGES and then find the individual to start a chat up. You'll see their name and avatar in new chat history which just starts off saying "This is the very beginning of your direct message history with Yuri Namehere." and you should click on the avatar as it will bring up a summary for the person at the right which will have a "Call" button. Click the "Call" button.