The situation is,
I did not add any phone number for my steam account before. While I open my steam app, it asks me to add a phone number to activate steam guard. But I cannot receive the verify code by SMS.


And here is my solution,
please do that on your desktop(cause I try that at the first time on my desktop and it worked. And never have chance to do that again on my phone)

Change the language of your steam application to the region your telecom belong to. Also, the prefer display language in your account.

And then, you can add a phone number successfully into your account.(Can receive the verify code by SMS)


(Here is the steps on your phone)
In the end, follow the instructions in the steam app on your phone to activate your steam guard.


Hopefully the developer could see this discussion. Looks like the sending SMS function try to get culture by the language of the application rather than the country code before the phone number......