“We saw that when people are speaking on the phone, typically they were doing something else,” said Ariel Maislos, head of the system's developer Pudding Media.

“They had a lot of other action, either doodling or surfing or something else like that.

"So we said, ‘Let’s use that’ and actually present them with things that are relevant to the conversation while it’s happening.”

The system is similar to the Adwords service that has generated millions of dollars in revenue for Google.

Whilst Google targets ads based on web searches and text in web pages, Pudding Media are using voice-recognition algorithms to pick out keywords in conversation and serve up ads accordingly.

The software is still in beta and is not always accurate.

"Sometimes crazy things pop up. It actually enriches the conversation, which is very cool," said Maislos.

Potential advertisers have given the development a cautious reception, as concerns over privacy refuse to go away.

“We can never obtain too much information from the targets, and I would love to get my hands on that information,” said Jonathan Sackett of Arnold Worldwide, part of the global marketing and advertising network Havas.

“Still, it makes me caution myself and caution all of us as marketers.

"We really have to look at the situation, because we’re getting more intrusive with each passing technology.”

Pudding Media claim that they do not store phone calls themselves or keep records of keywords used by individual callers.

"Have you talked about mountain biking? We wouldn't know," Maislos confirmed.