If youu2019re using an iPhone, your device will of course know its own number, since it is still a u201cphoneu201d. And when you change your phone number the iOS will automatically update your personal contact data, which will be synced to iCloud by default.It doesnu2019t matter whether youu2019ll have to exchange your SIM with a new one or not (if your service provider support that), your phone will need to sync telecommunication status with the phone company as part of the standard of GPRS/3G, thus it will know your phone number.If you donu2019t have an iPhone, Apple would only know it if you telling them, by updating your personal contact data in u201cContactsu201d. Itu2019s also possible that youu2019ve synced your contacts among various services thus the rest of them will also be notified when you make the change somewhere else.