X is rolling out new features that allow users to make voice and video calls. When users open the app after receiving this update, X will display a notification message.
Voice and video calling are some of the features that Elon Musk mentioned adding to X some time ago. To enable this feature (if you have received the update), go to Settings > Privacy & Safety > Direct Messages > Enable Audio & Video Calling. Then, choose the criteria for who can make voice and video calls, such as only people in your contact list, those who follow you, or accounts that are verified (indicated by a blue checkmark). As for making calls, you can do so from the DM page of the specific user.
Since this feature is being rolled out gradually, not all users may have received it yet, and there is no confirmation whether it is exclusive to X Premium users or not.
TLDR: X is adding voice and video calling features. Users can enable this feature by going to the settings and selecting who can make calls. It is unclear if the feature is available to all users or exclusive to X Premium.