As crazy as it seems at first glance, a double Duplex system would be a really beneficial result.
What stops businesses from setting up Apis from scheduling services today? It's the lack of a universal API.
If a barber shop wants to make it possible for a 3rd party app to book appointments then they have to release some API. But that's not the end of it. The 3rd party app has to first discover their Api, someone has to understand it and write code to use it, and then deploy that code.
I'll repeat for emphasis: This is mainly a problem today because there is no universal Api that all services can use
With Duplex, verbals speech becomes a universal Api that every service can parse and communicate to each other with. Also, the discoverability is taken care of by using publicly cataloged phone numbers on services like Google Maps, Yelp, etc
The problem of universal API is entirely orthogonal to voice communications. Duplex is not a Turnig-complete system, it's just an API behind a voice recognition layer. All the important problems for universal APIs happen after that layer.
Ultimately, what you describe can work perfectly only when everyone is using Duplex, which is equivalent to everyone using Google-defined API. That's not universal, because you have one entity behind it.
The only way this brings us somewhat closer to universal API is that if you expect it to handle humans as well, it introduces some constraints to the space of possible APIs, which could make it easier for everyone to agree on a common format. Constraints of natural language processing without a human-level AI require your API to be very fuzzy and very lenient. There's nothing stopping one from implementing those same constraints over a text or binary protocol. Nothing except no reason for businesses to do it.
Everyone doesn't have to use Duplex, everyone would have to use a system that allows it to seem like a human is talking to the other person, provided you limit the context to a particular domain (like taking reservations).
This system could be developed by any company with sufficiently advanced ML chops
What stops businesses from setting up Apis from scheduling services today? It's the lack of a universal API.
If a barber shop wants to make it possible for a 3rd party app to book appointments then they have to release some API. But that's not the end of it. The 3rd party app has to first discover their Api, someone has to understand it and write code to use it, and then deploy that code.
I'll repeat for emphasis: This is mainly a problem today because there is no universal Api that all services can use
With Duplex, verbals speech becomes a universal Api that every service can parse and communicate to each other with. Also, the discoverability is taken care of by using publicly cataloged phone numbers on services like Google Maps, Yelp, etc