After a call is answered, CallHub listens for 5 seconds to determine if a human or a machine picked up. If a machine is detected, then it waits for 2 seconds of silence. 2 seconds is silence indicates that the initial greeting in the answering machine is complete and the machine is waiting for the messages. CallHub will then play the audio file into the answering machine.
Answering machine detection is not accurate 100% of the time. It is accurate at about 60-70%. The failure rates are higher on answering machines with a initial greeting message less than 5 seconds long. The failure rates are also higher for cell phones because the voicemail on a cell phone is handled by the telecom carrier.
Voice broadcasting is better suited for calls that are optimized for a person answering the phone.