Send a short message to get the verification code. When can I receive it?

What verification code did you send? QQ security certification? Or open some service certification? Generally speaking, there will be a reply immediately after it is sent out, because these are all system operations. But I recently encountered a situation. I didn't respond to the authentication information sent by Unicom for a long time, and then it was ok to resend it. The information was returned together with the first article. However, in general, it will be received immediately. If it is not received, it will be almost the same if you resend it again!