According to Bloomberg, Facebook is letting some users in select countries (including the US) try out voice and video call features using the Facebook app. This would typically require going through Facebook Messenger, since Facebook separated the functions into the messaging app back in 2014. Though if you use Facebook through your computer’s browser, everything is still contained in one place.
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As of now, Facebook only has confirmed testing voice and video calls in the main app. It has not said anything about whether or not we might see other Messenger functions appear in the main app in the future. It also hasn't explained what might happen to the Messenger app if these features are brought back to the Facebook app. For the time being, at least, it wants us to keep using the Messenger app.