WE ALL know about the “one phone call” supposedly allowed to arrestees. Most of us are less familiar with the price gouging by prison phone service providers that renders the many calls inmates make over the course of their time behind bars a huge burden. The Federal Communications Commission was fighting in court to cap rates on those calls when they occur within a single state — until February, when Chairman Ajit Pai decided to drop the defense.

Even a believer in free-market efficiency can recognize that, in this case, the market has failed. Because prisons receive commissions from the phone companies they contract with, facilities strike deals with providers not to get inmates the best service at the best rate, but to get themselves the biggest kickback. Mr. Pai has acknowledged as much in his discussion of the FCC’s less controversial efforts to regulate rates on calls made between (not within) states, which the agency will continue to defend. Yet the problem is the same inside state lines as across them. And the FCC’s regulation of interstate rates has prompted providers to push intrastate rates even higher.

That leaves people such as Joanne Jones struggling to find room in their budgets to talk to loved ones who have been shipped hundreds of miles away. Ms. Jones tallied in 2015 that she had spent more than $1,000 over the past year on the line with her son in Texas. In some states, a 15-minute phone call can cost as much as $15 or $20 . It likely costs a company no more than 5 cents per minute to transmit that call , and probably less, according to FCC filings submitted by Lee G. Petro, the pro bono counsel for inmate advocates.

Phone companies say they need the money to offer their services securely and still stay afloat. One of those companies, Securus Technologies, was purchased for $640 million in 2013. This month, it was reportedly in talks to sell for almost $1.5 billion — hardly the mark of a business just scraping by. Prisons say they need the commissions for perks for inmates such as educational programs . But, Mr. Petro’s filings show, in many cases the cash is used for anything from guard duty to HVAC repair, or goes to states’ general funds instead. Besides, regardless of where the money ends up, it should not come out of the pockets of prisoners’ families.

The U.S. Court of Appeals for the D.C. Circuit heard the rate-cap case in February, even with the FCC refusing to defend its own regulations. Mr. Pai is right that the agency stands on shakier legal ground regulating intrastate rates than interstate ones . The court could still decide that the FCC can limit costs on both, in which case the agency should move quickly to enforce the proper price point. If the court denies the FCC that ability, it will be up to states to determine fairer regimes. But someone must do something: Inmates and their relatives literally cannot afford the alternative.