While the US has seen a vibrant debate around the use of IMSI catchers (commonly known as Stingrays), the position of UK authorities has been to consistently neither confirm nor deny use of the technology.
A heavily redacted report from the Scottish Prison Service (SPS) shows that two prisons have trialled IMSI catchers. In all, £1.2m ($1.8m) was spent on the pilot project, according to The Ferret.
Tom Fox, head of corporate affairs at the SPS, told Motherboard in a phone call that IMSI-catchers were being used "in a number of locations."
IMSI catchers work by mimicking a cell tower, forcing nearby phones to connect to them. The devices are capable of tracking phones as their owners move around, and some versions can also intercept text messages, listen in on calls, or block communications.
The SPS has been using the IMSI catchers to stop inmates from communicating. In this case, the system was commissioned to block 2G and 3G signals, according to The Ferret. However, prisoners apparently found a way around the system—although the documents don't reveal how.
"We gather information soley for the identification of the telephone number and SIM number of devices in use, with a view to having these devices disconnected," Fox told Motherboard. "We do it in a number of ways, and that is one of the ways that we do it."
"Some of the equipment is fixed, some of it is mobile," he added.
Originally, the SPS wanted to trial IMSI-catchers at HMP Edinburgh, but concluded that the risk of tampering with the phones of passers-by was deemed too high, The Ferret adds. So the pilot was carried out in two rural prisons instead.
Traditionally, UK authorities have declined to release much information, if any, on IMSI catchers. For example, in 2014 the Metropolitan Police service wouldn't even acknowledge it holds any information related to Stingrays, despite reports that the agency had purchased such a tool in 2011.
"We have been very careful not to say precisely where, or precisely the number of locations [that IMSI-catchers have been operating]," Fox said.
"We are catching people who are breaking the law, because they shouldn't have a mobile phone in prison; it's a criminal offense," he added.