Error message: Unsupported destination country

I want to test sending SMS messages to the mobile phone number, so I need to add the mobile phone number to the sandbox first. When I used the Chinese mobile phone number to add it to the sandbox, the addition failed, and the error message was as follows:

An error occurred while attempting to add a phone number to the SMS sandbox. The phone number was not added. Error code: InvalidParameter - Error message: Invalid parameter: Unsupported destination country