They make a request to twilio and their logic checks for a failure response instead of looking for a success response. They are not getting back a different failure response and this causes the requests to be incorrectly be marked as success.

For example here is how something like that might break:

    /auth 
    -> returns {"success": true} and 200 on success
    -> normally returns {"success": false, "error": "BLAHLBAH"} and 200 on failure

developer checks if response["error"] is null to check if it was successful or not

after your account is disabled /auth starts returning empty json: {} with a 500 error code

oops. now all the requests are incorrectly marked as success. i know this happens because i've seen similar things happen in real life.

the error could also be more obvious and the code just fails open. someone has a try/catch and returns true either because of an accidental mistake or because they don't want users to be locked out when the provider is down.