When Twillio is used for receiving the voice call alerts
Site24x7 will send the monitor details to Twillio, which will be forwarded as voice call alerts to customers from Twillio. But the data will not be read by both Twillio as well as by Site24x7. When Site24x7 calls the Twilio API then the Twilio system will know if the audio is being transferred or not. But Site24x7 or Twilio generally do not take any action based on the content of that audio.
Hence, Twilio considers a call to be completed when audio data is returned, irrespective of whether the phone number is switched off or not reachable or whether the audio data is an out-of-service message .
Also, when the mobile service provider accepts the call, their internal business logic determines that it should play an error message to the caller instead of ringing through. If that's the case, Twilio marks the call status as "Completed," and Site24x7 is charged. Hence Alert credits will be used even if the call is not answered, or when the number is not reachable or switched off.
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