How many authentication attempts are allowed before an extension is blacklisted?

The correct answer reflects the configurations typically seen in the 3CX system regarding security measures for extensions. In this context, once the specified threshold of 25 unsuccessful authentication attempts is reached, the extension becomes blacklisted. This feature is designed to enhance security by preventing brute force attacks, where an unauthorized user would try to guess the password by making repeated login attempts.

Understanding the significance of this limit is crucial, as exceeding it leads to the extension being locked out and requiring manual intervention to re-enable access. This ensures that there is a protective barrier against potential attacks that could compromise the system's integrity.

The other options provide different thresholds that are not consistent with the known configurations of the 3CX system. This reinforces the importance of adhering to the correct limits in practice to maintain the security and performance of the communication system.