Unknown phone number

Turn your phone number on or off for your Mac

For Messages, choose Messages > Preferences, then click iMessage. For FaceTime, choose FaceTime > Preferences, then click Settings. Turn your phone number on or off: To turn on your phone number, select it and your Apple ID in the list.

Add or remove your phone number in Messages or FaceTime

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.