Thousands of mobile phones are confiscated in UK prisons every year and many more - smuggled in or thrown over the wall - go undetected.
They are a valuable illegal resource - costing between £400 and £1,000 just to borrow.
The government's National Offender Management Service (NOMS) seized 7,451 mobile phones and Sim cards in prisons in England and Wales in 2013.
Using them, inmates had "commissioned murder, planned escapes, imported automatic firearms and arranged drug imports", NOMS said.
"The problem is widespread."
Machine guns were smuggled into the UK by a prisoner organising the crime by phone from his cell.
Judge David Farrell QC called the "wholly inadequate" prison security that had allowed the crime a "scandal".
Inmates have run a cocaine ring, arranged the murder of a teenager as part of a feud and organised the killing of a gang leader - all from their prison cells.
The mother of an inmate in HMP Northumberland claims "the place is full of mobile phones".
"You've got people throwing mobile phones over the fences and then there are prisoners who have access to the grounds so they're bringing them in," she says.
Glyn Travis from the Prison Officers' Association (POA) says the jail is far from unique.
"Drugs and mobile phones are freely thrown into prisons" with delivery by drone "completely undermining the external security that protects the public", he says.
Sodexo, which runs HMP Northumberland, said "staff worked hard to stop illicit items getting into the prison using a range of technical and intelligence measures".
But the fact that so many phones make their way into prisons despite security precautions goes some way to explaining how hard it is to find and remove them.
The obvious solution, says the POA, is to make them unusable.
Mobile phone jammers or grabbers - which block signals or divert them away from their intended destination - are readily available.
Jamming/blocking: A signal is transmitted to prevent the handset from receiving its base station signal. All phones and Sim cards within the jammer's reach will be blocked, including those belonging to prison staff. The method is cheap and mostly effective. Interference caused outside the prison can, with care, be avoided, but this may add to the cost.
Grabbing: Phones are attracted to a fake network. It is selective - staff or nearby residents' phones can be put on an unaffected "white list". Success can be quantified - phones, and their owners, can be identified. Illicit phones can be monitored rather than blocked. It is more expensive than blanket jamming.
Operator disconnection: The 2015 Serious Crime Act introduced the power to force mobile phone operators to disconnect illicit phones. So far, the relevant regulations have not been enacted. Disconnected phones and Sim cards can be replaced and mobile operators may be unwilling to co-operate.
Stop and search: Visitors and staff can be searched for illicit phones. Cells and inmates can be searched to find those which are missed. Sniffer dogs can be trained to find mobile phones. Some phones will escape detection and new ones can be brought in to replace those confiscated
