You may need to link your Apple ID and your Phone Number in Settings -> FaceTime and Settings -> iMessage.
Thanks, however my phone is already set up correctly. This issue occurrs after the phone has been working for several days or weeks. Then all of a sudden it pops up with this message saying I need to add a phone number to my phone. No one has the PIN to my phone so they would not be able to make any changes to it while I am not looking. Again thank you for your time and trying to assist me.
Thank you for using the Apple Support Community. Due to the nature of your issue or question you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action.