When you think of all the things that prisoners aren’t allowed to have on them, cellphones might not top the list. They do for prison officials.

“Prison officials see this as the most dangerous form of contraband,” says Kevin Roose, senior editor at Fusion, whose ongoing three-part story explored how prisoners use technology.

U.S. prisons, says Roose, are spending millions of dollars to keep cellphones out of prisons, mostly because the phones have been used to harass victims and coordinate crimes from behind bars.  

Now prisons are trying out something called a “managed access system.” It acts like a cell tower, and intercepts data and calls from the person using the phone before they reach the carrier. If you are not authorized to send or receive calls or texts, you’re blocked from doing so. Managed access systems can cost up to $1 million, but many correctional systems think they have a better chance at blocking the phones than keeping them out of inmates’ hands.

Phones are usually smuggled in by guards who, according to Roose, charge prisoners “hundreds or thousands of dollars apiece.” Soon, an “underground economy” crops up: an inmate who has a phone starts renting it out to others.

Not everyone in prison is using cellphones for the same purpose. In some cases, Roose found, people just wanted to find a way to “stave off the loneliness.” Like one man who makes six-second videos for Vine and has a couple of hundred followers.