Open the SMS application, press the menu button (at the bottom of the screen, some models need long press), and then select Advanced Settings-Set SMS Center Number.
The paths of some MIUI systems are: Settings-System Applications-SMS-Settings-Advanced Settings-SMS Center Number.
The number of China Mobile SMS Center is +86 1380ABCD500, and ABCD is the local area code. If the area code is three digits, the last digit should be 0. (Example: Shanghai 02 1, and the number of SMS center is+8613800210500; Zhengzhou, Henan 037 1, SMS center number +86 1380037 1500)