You can have a look at this video on how to receive a call in Slack: https://www.youtube.com/watch?v=b9rAEfRj6_U.
Before you can receive calls on your Yodel number, you firstly need to check if it's configured correctly.
Please go to manage.yodel.io and select your number on the left side in the menu bar. There, you need to select a Slack channel from the drop down menu. Once this is done, you can receive calls in Slack.
Call your Yodel number and answer the call in Slack. Simply click on "click to answer" to start the browser.
The browser opens the Yodel call interface and you only have to click on the green button to start the call: