We are using a Call Progress Analyzer (CPA) when doing answering machine (voicemail) detection. While there is no consistent signalling difference between a call picked up by a human or a machine, we detect voicemail using heuristic algorithms which are based on tone of voice, speed of spoken word etc. This detection usually takes 3 to 5 seconds and ClickSend's machine detection typically has an accuracy which is above or on par with the industry standards.
The time used for analysis with our Voice API is 5,000 milliseconds (or 5 seconds). If a machine is detected during the this time, we wait a bit longer before starting to read out the message. We also record that it was an answering machine which shows on reports and can be accessed via the API delivery reports.
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