You can have a look at this Video on how to make a call in Slack: https://www.youtube.com/watch?v=a4Ap_q6bQFc
After completing the onboarding, you will have a Slack integration set up and a phone number in place. To now start a call, go to any of your Slack channels and type in the command /yodel.
You will then see a list of possible commands:
Type in the number you want to call, including the country code (e.g. for US, you need to type +1) and hit enter.
Click on "Start call" beneath your phone number to open the Yodel Call Interface.
By hitting the green handset symbol, the call gets initiated.