Facebook's Messenger is easily one of the most popular messaging apps available on mobile devices, and just like many of the top messaging options that are out there it offers up quite a few features to really make it a robust application. One of those features is the option to place calls. Users are able to make voice calls as well as video calls to any of their contacts in the Messenger friends list, and it would appear that Facebook wants to highlight the call feature a little more as they're now testing the use of a dedicated calls tab at the top of the app.
Whether or not they end up keeping the tab is still unclear, as they could test it and find out that it wasn't helping users make more calls which is likely the reason for putting the tab front and center, so users would notice it more. With a tab right at the top, placing a call to any contacts would presumably be much simpler for most people, and it would give users a way to see a list of recent calls that they have placed earlier much like you would find in a regular phone application.
If Facebook's Messenger app is your primary form of making phone calls, then having the tab within the app would likely be a very useful element, but keeping it most likely hinges on whether or not people use the call feature enough to change things up from the norm. Adding a dedicated call tab is not the only new change thatFacebook has been making to Messenger lately, as you can now check in for your KLM flights through the app. Facebook is also rumored to be toying with the idea of adding a "secret message" option for users who want their conversations to be more private. In the wake of privacy being a valued trait these days, it shouldn't surprise anyone that Facebook would want to give this type of feature to their users, although it's worth mentioning that the feature may actually never surface as something that's available.