To make a 1:1 voice or video call, navigate to the conversation stream of the person you wish to contact. You can do this either by finding the conversation in the Recents tab of the application, or you can search for the person in the Directory tab and click their name.
If you have not previously had a conversation with the person you wish to call, you will need to start one (learn about starting conversations here).
Once in the conversation stream, there are two icons in the top right corner that offer different calling functions. Tapping the phone icon allows you to make a voice call.
Tapping the video camera icon directly next to the phone icon will connect you to a video call with the other person in the conversation stream.
Note: This is for 1:1 voice and video calling help. To learn about group calling, click here.