When Google launched Duplex with a demo at I/O last year, the audience was left wondering how much of the call was staged. The AI-based reservation booking service seemed almost too impressive to be a machine. Now that it’s been used for real-world reservations, Google has revealed that it frequently isn’t.

The company recently told The New York Times that Duplex calls are often still made by human operators at call centers. Roughly a quarter of calls start with a live human voice. Of the calls that start with machines, 15% require a human to intervene.

Google told us during a demo last year that humans would be monitoring the system, ready to take over if something went haywire. That’s to be expected, of course. This sort of real-world testing ran into some snags as the company works to iron out the kinks, now that the product is available for both iOS and Android devices. But the 25% initiated by people seems a little high for the advanced AI-based system.

Along with initial test driving, Google is very much in a period of data collection for the service. While Duplex is extremely impressive in fits and starts (I’ve tried it, and it’s capable of fooling the listener for a quick reservation, if all goes well), the neural network requires a tremendous amount of data to improve, even though it’s essentially limited to a single task.