Answering Machine vs. Human -- Neural Network Features Selection
I don't think that you would do this by feature extraction. Instead, I would train a many-to-one LSTM, which classifies the input sequence as -1 or 1. For you, it is easy to collect a huge amount of labeled speech data, just by calling. Note that, the data size is important. 

LSTMs are very powerful techniques and should handle this problem gracefully.