When using answering machine detection, there's certain steps you can take to give Tropo the best chance of making an accurate detection.
Set the introduction parameter. Without it, the person answering the phone will hear silence while Tropo determines if they were a human or machine.
For the most accurate results, the "introduction" should be long enough to give Tropo time to detect a human or machine. The longer the introduction, the more time we have to determine how the call was answered. If the introduction is long enough to play until the voicemail "beep" plays, Tropo will have the most accurate detection. It takes a minimum of four seconds to determine if a call was answered by a human or machine, so introductions four seconds or shorter will always return HUMAN.
If it's important to absolutely know if the answerer is a human or machine, then after Tropo detects HUMAN, use Tropo's ask() method to ask the person to press a key. Because machines can't respond to the question, an answer will confirm this is a machine.