In prisons and jails across the country, a bulky tablet enclosed in a screwed-on plastic case has become the hottest new device.

Featuring limited online access, the tablets allow incarcerated people to make calls, send messages and watch movies from their cells. They also give prison telecommunication companies and correctional facilities another source of revenue when profits from phone calls, which have long been the industry's principal business, are getting squeezed.

In July, the Federal Communications Commission voted to slash the rates and fees that companies can charge for prison or jail phone calls and impose price caps on previously unregulated video calls. The rules, which will take effect next year, also outlaw site commissions, which are clauses in contracts that give a correctional facility or agency a portion of the revenue from calls.

At large jails, a 15-minute phone call that previously could have cost more than $11 will now be at most 90 cents. The new caps will save incarcerated people and their friends, families and legal teams $386 million, according to the FCC.

The new FCC rules regulate voice and video calls-whether they are made through wall phones or tablets-but not other services available on tablets, which companies have distributed in hundreds of jails and prisons over the last decade. Educational content on the devices is usually free, but e-messaging, music, ebooks and movies can cost prisoners significantly more than they would outside of confinement.

While the FCC decision will prohibit site commissions from voice- and video-call revenue, correctional facilities and agencies can continue receiving portions of revenue from the e-messaging, entertainment and equipment that companies sell through their tablet programs.

Securus Technologies and ViaPath Technologies, the two biggest providers of prison phone and tablet programs, say that by providing safe access to digital services, they enrich the lives of incarcerated people and improve the environment within facilities.

Some advocates for prisoners say that tablet programs, like the phonecalling services before them, are exploitative because they charge high prices to incarcerated people, who often have almost no income and cannot choose between providers.

Companies are "getting these monopoly contracts," said Paul Wright, executive director of the Human Rights Defense Center, a prisoner advocacy group in Lake Worth, Fla. "And that is that. The consumer, the prisoner, has no choice."

The companies say competition is achieved through the bidding processes facilities hold to select their provider.

Letters out, tablets in

Michigan's St. Clair County Jail had banned face-to-face, in-person visits five years before Tray Proch arrived in 2022 to begin his monthslong sentence, meaning one of Proch's only ways of maintaining contact with his family was via the jail-issued tablets. It cost him $5 a month to rent his own tablet, $4.20 to make a 20-minute phone call and $12.99 for a 20-minute video call.

Soon, citing the risk of contraband smuggling, the jail banned most physical mail, too, which Proch had relied on to receive photos and drawings from his children. That meant he had to use the e-messaging service on his tablet. Each message cost a 50 cent "stamp," each attachment required an additional stamp, and buying stamps brought a $3.75 transaction fee.

"It's not the tablets that are the problem," said Proch, 42, who now works as a home remodeler. "My issue is how they were wielded. They were used to extort people."

Because of the new FCC rules, St. Clair County will soon have to drastically lower the price of its phone and video calls. But the price of a text or a movie won't have to change.

St. Clair County receives commissions from the tablet program, which is operated by Securus. Proch and legal advocacy groups are suing the county and the company, alleging they conspired to force incarcerated people to purchase Securus services.

The county said the mail policy has been effective at reducing the influx of contraband, while Securus denied any conspiracy to enact the mail policy.

Securus and ViaPath have for years supplied phone services to hundreds of correctional facilities and have begun to provide tablets to many of them. Both companies recently refinanced more than $1 billion in debt, originating from leveraged buyouts by their private-equity owners. In July, S&P Global downgraded ViaPath's debt rating.

Regarding its phone-call service, a ViaPath spokesman in July said that it offered "the lowest rates in the industry across the country-often already below FCCproposed limits." A Wall Street Journal analysis of the company's annual report filed in April found that more than 80% of the jail and prison systems where ViaPath provided service last year had 15-minute call prices above the new maximums.

Free phones, expensive extras

In Colorado, where a state law passed last year will soon make prison phone calls completely free, the Department of Corrections implemented a Securus tablet program in July, offering paid messaging services and entertainment. New York City, which in 2019 became the first major city to make jail phone calls free, agreed to a contract with Securus this year to allow the company to sell premium content to prisoners through tablets.

Under the New York contract, which was viewed by The Wall Street Journal, purchasing a song can cost up to $2.50 and a movie rental can cost $2 to $25. A "gold package" subscription with 100 newly released movies and TV shows costs $21.99 a month, while a separate monthly music package costs $24.99.

Securus has "deployed 600,000 tablets to help bridge the digital divide, creating safe, affordable, and sustainable connections that would otherwise not be available to incarcerated individuals," it said.

Tablet programs can also increase revenue for correctional agencies, many of which say the new FCC rules regulating phone calls squeeze them financially. In addition to the end of site commissions from phone revenue, which paid facilities or agencies $446 million in 2022, the new FCC rate caps don't incorporate the costs of some security services companies offer, including surveillance on calls. Now, instead of inmates paying for those services through their phone rates, facilities will have to foot that bill.

Tablet programs can also increase revenue for correctional agencies.

"There is not an unlimited pot of money. At some point, something has to give to cover these costs," said Shawn Laughlin, president of the American Jail Association, an organization that supports local correctional facilities.

Phone calls from facilities in Connecticut became free in 2022, a change that the state said would cost about $5 million annually and reduce revenue by about $6.5 million from lost commission payments. The expansion of the state's Securus tablet program has made back part-but far from all-of the money.

According to records requested by the Journal, Connecticut earned about $1.1 million in 2022 and 2023 from tablet commissions, up from about $220,000 in 2021. A Connecticut DOC spokesperson said the funds are used for inmate programming.

One city is pushing back on tablet prices. San Francisco in 2020 made phone calls in its jails free and introduced in 2023 a tablet program with free educational and entertainment content. The city's library system pays about $5,000 to $10,000 a week to provide books, music, movies and shows accessible on the tablets.

David Thornton, who is incarcerated at the San Francisco County Jail, said the program is immensely helpful to individuals' mental health. "A lot of people are here because of financial struggles," he said. "If they have to come in here and struggle all over again ... that's an unfair program."

Write to Nicholas G. Miller at Nicholas.Miller@wsj.com