https://www.theguardian.com/technology/video/2018/may/09/new-google-assistant-mimics-human-voice-video

Google’s CEO, Sundar Pichai, delivers the keynote address at Google's 2018 conference. The robotic assistant, Google Duplex, uses a very natural speech pattern that includes hesitations and affirmations such as “er” and “mmm-hmm” so that it is extremely difficult to distinguish from an actual human phone call