The operation path may vary by app version.
China Mobile:
Online service center: Go to the China Mobile Official Website, log in with your phone number, and take a screenshot of your phone number and its registration time.
App: Go to the app of China Mobile and choose Me > Profile picture to view the phone number and its registration time.
China Unicom:
Online service center: Go to the China Unicom Official Website, log in with your phone number, choose Services > Inquire > My basic information, and take a screenshot of your phone number and its registration time.
App: Go to the app of China Unicom and choose Me > Profile picture to view the phone number and its registration time.
China Telecom
Online service center: Go to the China Telecom Official Website, log in with your phone number, and take a screenshot of your phone number and its registration time.
App: Go to the app of China Telecom and choose Me > Profile picture to view the phone number and how long it has been used.