There are a few ways to initiate a call:
Within a one on one conversation, multi-participant thread, or a group chat: click the 3 dots at the top of the chat to show more options. Select the audio or video call to initiate the call.
Tap on the contact the Group name at the top of the thread to open the Contact Info or Group Info. Select the audio or video call to initiate the call.
From your feed (desktop): Hover over the message and click the 3 dots that appear to show more options. Select the audio or video call to initiate the call.