Currently for WebEx Edge customers there is no dynamic way of blocking the fraud calls which are retried via PSTN audio.
Our Customer have added a search rule on the expressway which can script, identify fraud calls and make dynamic decisions as to what to block.
But as per current design :
Calls rejected from enterprise due to Toll Fraud is expected to return ‘403 Unknown Webex Site’.
Other rejections due to search rules may send ‘404’ back to Webex.
On both of these cases, WebEx will attempt retry via PSTN if the retry flag is enabled by the Site Admin.
Customers would not be able to disable “Retry call using PSTN Audio" for genuine fallbacks mechanisms and can't use the "Webex allowed Callback Countries" block list as it does not allow them to dynamically [vi API] block countries or individual prefix's within a country dynamically.
This request came from our premium customer who have requested product team to work on a feature for WebEx edge customers, so they can send back response that suppresses a PSTN retry for fraud calls.