Do I need to add a country code to send a text message to a domestic number?

No, the instructions for using the SMS service are as follows: 1. Sending short messages to domestic numbers can be directly sent to "/kloc-0 133XXXXXXXX" (this method includes sending short messages when roaming internationally). 2. The number sent to the roaming place, such as Hong Kong, needs "00852XXXXXXXX". 3. The format of sending international short messages is: 00+ country code+user number or 00+ area code+user number. The SMS function is turned on by default on the mobile phone, so it doesn't need to be turned on separately. SMS is valid for 48 hours. For more service benefits, click on "official website" customer service 222 below to answer your questions.