Why can't I receive SMS verification code abroad?

Take Unicom operators as an example. The solution of not receiving SMS verification code abroad: 1. Log in to Unicom official website, and answer the phone (the user name is the mobile phone number, and the password is the service password needed for the telephone service. If the service password has never been set, it will still be the initial password with 6 zeros). 2. Call Unicom customer service. Press 0 to pick up the labor and open roaming service (no monthly rent, subject to actual calls and text messages). If you just want to receive short messages, you don't need to open Internet packets). 3. After about 10 minutes, the customer service will inform you that the roaming function of the domestic mobile phone number has been opened in the form of callback, and the domestic mobile phone will receive a SMS reminder. Then you can receive the verification code in real time. CAPTCHA is the abbreviation of "automatic public Turing test to distinguish between computer and human", and it is a public * * * automatic program used to distinguish whether the user is a computer or a human. It can prevent: brushing tickets, flooding forums, and effectively prevent hackers from constantly trying to use specific programs to violence specific registered users. In fact, using verification code is a popular way for many websites now, and we have realized this function in a relatively simple way. This question can be generated and judged by computers, but only humans can answer it. Because the computer can't answer CAPTCHA's question, the user who answers the question can be considered as human.