Prison staff are using technology to find and seize phones used illegally by inmates in England and Wales.
New detection kits can narrow a phone's location down to a single jail cell, the Ministry of Justice said.
Staff get an alert when a phone is detected, which helps them track inmates organising drug smuggling or contacting criminals on the outside.
After a six-month trial at one jail, the kits will now be used at four more. The locations are not being revealed.
The real-time alerts are shown on a digital heat map which identifies the strength of the signal.
The results can be used as evidence in police investigations and can lead to arrests, the MoJ said.
'One step ahead'
Justice Secretary David Gauke said use of the technology was "vital" to make prisons places of "safety and rehabilitation".
"As criminals look for new ways to smuggle contraband into prisons, it is vital that we stay one step ahead, and this kind of technology will help prevent them operating from their cells," he added.
At least 15,000 mobile phones or SIM cards were confiscated in English and Welsh prisons in 2017 - equivalent to one for every six inmates.
This new technology is not used to block illegal mobiles remotely.
Under the Serious Crime Act 2015, all prison governors in England and Wales can seek a court order to completely remove a mobile or sim from a network.
In Scotland, prison authorities can use technology to block phones remotely before seeking to block them from a network.