The invention features systems and methods for detecting and mitigating network attacks in a Voice-Over-IP (VoIP) network. A server is configured to receive information related to a mitigation action for a call. The information can include a complexity level for administering an audio challenge-response test to the call and an identification of the call. The server also generates i) a routing label based on the identification of the call, and ii) a script defining a plurality of variables that store identifications of a plurality of altered sound files for the audio challenge-response test. Each altered sound file is randomly selected by the server subject to one or more constraints associated with the complexity level. The server is further configured to transmit the script to a guardian module and the routing label to a gateway.