How does it work?
The official support page says that you are able to control your audio and video calling options from the Direct Messages settings. From there, you will notice that by default, you are able to receive calls from accounts you follow or have in your address book — provided that you give X access to your address book. However, in order to call another user, both the caller and the recipient must have been in contact at least once via X’s Direct Messages (DMs).
X’s Audio and Video call settings in Direct Messages
The updated calling features aren’t instantly available for everyone. X is slowly rolling them out, likely in batches, and it appears that they are automatically turned on without prompting. If you do not wish to make and receive calls on X/Twitter, you may need to log in to the app and navigate to your Direct Message settings to turn the feature off.