What does it mean when Firebase phone authentication fails to work due to the absence of SMS?

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When Firebase phone authentication fails to work due to the absence of SMS, it means that the authentication process is unable to send the verification code to the user's phone number as a text message. This often happens due to several reasons, such as a weak internet connection, incorrect phone number input, or a problem with the user's phone carrier. As a result, the user cannot complete the authentication process, and the app or website cannot verify the user's identity. In such cases, the user may need to ensure that their phone number is correct and that they have a stable internet connection before attempting the authentication process again.