The technology can not distinguish if a passenger or a driver of a vehicle is using a phone and so the sign will be activated regardless of who is using the mobile.
It works by detecting if there is an active 2G, 3G or 4G signal in operation. When the device, created by technology firm Westcotec, spots a phone being used, it will flash a mobile phone symbol at the vehicle to advise the driver to stop using their mobile phone.
Police have unveiled an all-new device which is able to detect when a driver is using their mobile phone while behind the wheel of their auto.
It will allow TVP and Hampshire to target hotspots and use a sign to tell motorists to stop using their phones.
The campaign is being supported by Kate Goldsmith, who lost her daughter Aimee after a lorry driver crashed into the auto she was a passenger in while he was using his mobile phone to change music while driving.
In a joint operation with Thames Valley the Roads Policing Unit will be the first in the country to utilise new technology to help reduce the amount of people using their mobile phones whilst driving.
The mother of 11-year-old girl killed on the A34 in 2016 is backing the use of the new technology.
He was sentenced at Reading Crown Court after pleading guilty to four deaths by unsafe driving and a single count of causing serious injury by risky driving.
Pete Williams, RAC road safety spokesperson, said: "Driving and using a handheld phone do not mix, it is an incredibly unsafe and distracting combination". They will then be moved between different locations throughout the Thames Valley and Hampshire.
Anyone caught using a mobile while driving can get a £200 fine and six points on their licence.
If people are using a Bluetooth hands-free device, the detector will recognise this and not flash.
"It is also apparent that you are twice as likely to be involved in a fatal collision when texting compared with drink driving".
"I have stopped a few people and said, "you're using a mobile phone - it's actually a driver like you that killed my daughter", she said.
"Additionally, officers will be carrying out enforcement activity throughout next week".
"My advice would be to turn off your phone whilst driving, put it out of reach, out of view so that more innocent people don't lose their lives".
The technology will not be used as an "enforcement tool", the forces said, but was instead aimed at educating motorists and identifying offending "hotspots".
Norfolk County Council was the first to trial the road signs past year, but this is the first time that the police has made a decision to deploy the technology.
Thames Valley Police and Hampshire Constabulary have developed the technology with Westcotec Ltd.