China Mobile does not open international roaming. Is there a signal to Hong Kong and Macau?

No matter what kind of user you are of China Mobile, as long as you don't have international roaming service, you can't use your mobile phone in countries and regions outside Chinese mainland, and it is impossible to detect the available signal network. Because when a mobile phone detects a signal, it needs to send a request to the switching center through the base station, and the switching center will only turn on the signal after verifying the right to use the mobile phone number. How can a mobile phone without the right to open international roaming have a signal and send a short message?