Google's remarkably human-like Duplex restaurant reservation service is one of the most impressive pieces of artificial intelligence ever created. But not all is as it seems.

Despite deeply impressive demonstrations of how the system, a part of the Google Assistant, can call up a restaurant and make a reservation on your behalf, it regularly relies on human help.

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• This is how Google's Duplex AI calling system works
• Could Google Duplex replace the call center with AI?

Google has admitted that a quarter of Duplex calls are started by a human in one of the company's call centers, and a further 15 percent of Duplex calls see a human intervene when the AI gets confused.

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This admission was made after the New York Times conducted its own tests of Google Duplex, and found the calls it asked the system to make were completed by a human 75 percent of the time.

This human hand-holding shouldn't come as too much of a surprise. Artificial intelligence requires a lot of training, and it learns best when shown what to do. In the perfect situation, Duplex should be able to call up a restaurant and make your requested booking - including the right date, time and party size - then send you a notification that it has done so successfully.

But if the person who answers the phone strays too far from the script expected by the AI, the system will struggle to make sense of the conversation. In these instances, a Google call center employee steps in to complete the reservation.

As part of its testing of Google Duplex, the NY Times used Duplex to call over a dozen restaurants over a two-day period, then reported: "...our tests showed a heavy reliance on humans. Among our four successful bookings with Duplex, three were done by people."

Many of the failures were due to restaurants only accepting bookings for parties of 10 or more people, not the two to four people the Times often wanted, so not entirely the AI's fault.

Duplex is a part of the Google AssistantGoogle

To Google's credit, the report added: "But when calls were actually placed by Google's artificial intelligent assistant, the bot sounded very much like a real person and was even able to respond to nuanced questions."

During the one call which was handled entirely by Google Duplex, the AI managed to answer the restaurant's unexpected question of "Are there any kids?" with: "I'm actually booking on behalf of a client, so I'm not too sure."

As well as using humans to help the AI when it struggles to understand, Google says these callers are also used to help it take a conservative approach and be respectful to business owners. Google say a human might also be used if the company suspects the caller is using Duplex to spam businesses with multiple calls.

Another stumbling point here is a lack of transparency when it comes to whether restaurants take bookings or not. When a user asks their Google Assistant to make a reservation for them, the system will first try to do so online through services like OpenTable and Yelp. If this isn't possible, Assistant will use the Duplex AI to make a phone call, and at this point a human may get involved to check telephone reservations are possible, instead of relying entirely on the AI.

Naturally, a complex AI system like this will require a lot if training to become accurate. This is what Google is doing, while also trying to not annoy businesses receiving these calls.

Then there are the ethical implications such a lifelike AI throws up. While this technology is clearly designed to help make our lives easier, it could also lead to people struggling to work out if they are speaking to a human or a machine.

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