Ask HN: How would I build something similar to Google Duplex?

Google Duplex makes calls for you, but with a natural-sounding human voice instead of a robotic one. It seems the target was originally restaurant reservations (i.e. controlled environment).

I want to make something similar but for my own use case (daily accountability calls for my clients). What APIs would you recommend I check out to do this (from controlled NLP to voice synthesis)?