Calls are currently only available with one recipient at a time. Group-calls or conference calls are not currently possible.
If the button is grayed-out, the other person cannot currently receive phone calls. This may be because they are offline or are running an older version of the app. [1] X Research source
If the video button is grayed-out, the other person cannot receive video calls. They may be offline or may not have Messenger updated to the latest version.
