The detector picks up 2G, 3G and 4G signals and will therefore flash to alert people in cars who are using phones to call, text or data.
If people are using a Bluetooth hands-free device, the detector will recognise this and not flash.
The technology cannot 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.
The forces say the two detectors, which cost £6,000 each, will be located on the A34 in Oxfordshire but will be posted at different locations throughout the Thames Valley and Hampshire to start – but more could be rolled out.
Drivers caught using a mobile phone while driving are currently fined £200 and given six points on their licence.
Matt Barber, deputy police and crime commissioner for Thames Valley, said the system was “not fool-proof”, but added the police needed to “make it as socially unacceptable to use your mobile whilst driving as it is to drink and drive”.
PC Liz Johnson, a roads safety officer, said research suggested a driver was four times more likely to crash if they were using a phone and twice as likely to be involved in a fatal collision when texting compared with drink-driving.
“It is vital that people take notice and stop using their mobile phones whilst driving,” she added.