It's worth noting that Google Assistant has been able to make reservations with OpenTable since 2014. No need to have Duplex talking to Duplex; it's just protobufs or JSON or XML or whatever people use nowadays for RPCs.
Google Duplex is about helping the long-tail -- it's work that is done by studying the needs and processes of the smallest of small businesses, and tailoring a product just for them.
I've lost count of the number of college hackathon projects where they say, "oh, push a button and you get a pizza" and they think they'll just put an iPad in the kitchen, and then fizzle out when they get to a real restaurant.
In practice, the restaurant might pass around pieces of paper in the kitchen. So you think, oh, I'll put in a thermal receipt printer. But then you realize that they don't have Wi-Fi or internet, so now you have to put in a $70/month internet bill, on top of the phone bill, and a router or two. So you think, "oh, I'll use a fax machine", or "I'll integrate with the point of sale system". But the fax machine runs out of paper, and the point of sale system is an offline piece of ---- running Windows XP. And even if you do get them using an iPad or OpenTable or Yelp or whatever, before you know it, you have waiters writing on a computer monitor with a whiteboard marker: https://javlaskitsystem.se/2012/02/whats-the-waiter-doing-wi...
But every one of these businesses has a telephone number, whether it's a landline or a cell phone or whatever.
When pg says to talk to your customers (https://twitter.com/paulg/status/898476047263518720), he means, talk to your customers. You'll be surprised by what you learn.
(Disclaimer: I work at Google, but on YouTube, not on this product.)