If you are getting the "Update Apple ID Phone Number" notification and it locks your settings app, do the following...
Close the settings app from the app switcher. Then go back into settings and turn off iMessage and FaceTime. Now, go into your Apple ID in the settings app, then password and security, and then edit trusted number. Just press add number and put your current one. You should get a text code, and because of iOS 12, it should auto fill the code for you. Then you just accept it and the message will go away. Then you can turn iMessage and facetime back on. That’s what worked for me!