Despite being unable to pick up whether the driver is using the phone, it is hoped it will act as a deterrent to people making calls behind the wheel.
The road signs work by using a scanner to detect the radio signals emitted when someone in the auto is connected to a call, with this data used to illuminate a sign further down the road.
Officials have worked with Dereham company Westcotec to develop the system which will flash a warning signal to drivers, similar to those for speeding, if a phone is being used in the auto.
People using their phones while driving will be getting warning messages if they do so in Norfolk.
The UK's first road signs which will electronically detect and warn drivers who are using mobile phones have been introduced. It was claimed that the technology, which is initially being introduced in Norfolk, could be adapted as part of a system to catch and fine motorists, although this is not possible at present.
Statistics from the devices will be shared with Norfolk Police, the county council said.
The system was developed by Westcotec, a Norfolk-based road traffic technology company.
"Hopefully, as time goes on it will become as socially unacceptable as drink driving".
"Any scheme which prevents this kind of behaviour is welcomed".
Chris Spinks, Specialist Product Sales, Westcotec "being a local company we're delighted to be able to trial this technology first within Norfolk, we've worked very closely with the road safety team at Norfolk County Council to get to this position and are glad that we're able to assist them in promoting awareness about the dangers of mobile phone use in the car".
"Using a phone at the wheel is one of the fatal four road offences which can have devastating consequences if it causes a fatal or serious collision".
"Whilst this is still not a ideal science, the new generation of sign is significantly more accurate and reliable than the first".
After an initial presence at Holt Road, the equipment will then spend a month at Cromer Road, Holt; Shipdham Road, Toftwood and Norwich Road, Attleborough.
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