(CNN) The sudden chirping of a cell phone sent Joel bolting out of his seat.
He rushed to a purple backpack sitting behind him and opened the front pocket.
An alarm was sounding on the phone US immigration authorities had given him, and he knew he had to act fast.
He fumbled with the phone for several minutes, struggling to understand the app's English-language instructions and follow its rules for snapping a selfie.
"Are they going to send me back to Cuba?" he asked, worried he'd be returned to the country he says he fled after facing police persecution.
Joel, who asked to be identified only by his first name to protect his safety, was among several migrants who spoke with CNN recently in San Antonio, Texas. He's one of a growing number of Cubans crossing the US-Mexico border and seeking asylum in the United States. And the government-issued cell phone he carried shows he's also part of another growing group.
Immigration and Customs Enforcement is monitoring nearly a quarter of a million migrants in a program using GPS ankle monitors, phones or an app known as SmartLINK, according to the agency's latest statistics. The Biden administration has rapidly grown the number of people in this program, known as "alternatives to detention," or ATD.
It's not clear how many migrants have been loaned phones as part of the program. ICE hasn't released that data in its regular public updates about the program, and the agency didn't respond to CNN's requests for those statistics. But lawyers and advocates who work with migrants told CNN the government-issued phones -- which can only be used with the SmartLINK app and can't make calls or access the internet -- are becoming increasingly common.
"We've seen a drastic, drastic increase in the use of this technology," says Javier Hidalgo, an attorney at the Refugee and Immigrant Center for Education and Legal Services (RAICES).
"This is an expansion of how DHS is defining detention," he says, "and there's a whole host of issues that come with it."
Officials argue these forms of monitoring are an effective way to manage cases. But critics on both sides of the immigration debate say the ATD program raises big questions that should concern every American.
The Department of Homeland Security, meanwhile, has defended and praised it. Alternatives to detention were mentioned as part of the government's plan for lifting Title 42, and DHS is asking Congress for millions of dollars to grow the program even more.
Alternatives to detention aren't new; ICE's program officially began in 2004 and officials began using the SmartLINK app in 2018. The agency relies on BI Inc., a subsidiary of private prison company GEO Group, to run it.
Now the program is expanding -- and fast. It's more than doubled in size since President Biden took office, according to an analysis of government data by the Transactional Records Access Clearinghouse (TRAC) at Syracuse University.
"It continues to show just a massive amount of growth," says Austin Kocher, a researcher at TRAC.
And all that growth, Kocher says, can be attributed to the increasing use of SmartLINK, an app that requires users to send photos of themselves as a form of checking in with authorities.
Currently more than 185,000 people are being monitored by SmartLINK -- about three-quarters of those enrolled in the ATD program, ICE says.
That's a steep increase from less than three years ago, according to Kocher's analysis, when SmartLINK monitored less than 6,000 people for ICE.
In April and May, Kocher says, about 1,000 people a day were being enrolled in the program.
ICE says using SmartLINK and other alternatives to detention "effectively increases court appearance rates (and) compliance with release conditions." Officers determine what level of supervision is required and what technology will be used on a case-by-case basis.
When migrants like Joel who are selected for the program are released from custody, ICE says some are loaned devices loaded with SmartLINK installed, unless they have their own smartphones.
The government-issued phones cannot make calls or be used to access the internet, ICE says, beyond using the app for its intended purpose. Migrants who have their own devices are asked to download the app onto their phones.
SmartLINK "uses facial recognition software to verify identity, GPS data point capture, push notifications and reminders, direct messaging with case officers and participants, and a searchable services database," according to ICE.
"Those who do not report," ICE says, "are subject to arrest and potential removal."
Joel told CNN he's currently required to send in a photo to check in weekly. The first time he tried to submit a photo, he received multiple errors, and -- at one point -- an ominous warning: "two attempts remaining." After four tries, his photo was accepted and he received a notification that the check-in was complete.
"It scared me," he said. "I can't go back to Cuba."
Hidalgo, who regularly works with asylum-seeking migrants, says many clients have recently reported receiving phones with the SmartLINK app installed.
But the phones, he says, are raising many concerns. Some migrants, he says, have reported difficulties with charging and powering on the devices.
"There's a fear...that folks are going to miss their check-ins just because of the technology issues," he says.
ICE says participants with concerns about monitoring technology can contact case specialists for assistance.
GEO Group declined to comment, referring questions to ICE. BI's website touts the "hassle-free setup" and increased battery life of its mobile devices.
"Clients can easily comply with supervision terms," an information sheet on the site says, "without the hassle of app stores and traditional smart phone functionality and updates."
A section about alternatives to detention on GEO's website says devices preloaded with SmartLINK aren't smartphones since they can't browse the internet, make calls, send or receive texts, or access app stores.
The company says it's a myth that the app frequently malfunctions and causes immigrants to miss check-ins. "An average of 88.4% of SmartLINK check-ins were completed successfully over the last five years," GEO says.
Immigrant rights advocates argue that expanding the alternatives to detention program is creating new problems and raising major privacy concerns.
Several organizations -- Mijente, Just Futures Law and Community Justice Exchange -- filed a lawsuit in April asking a judge to order ICE to release more information about how data obtained through SmartLINK is tracked, used and shared. The suit decries "constant, invasive surveillance" and says the program takes a major toll on the immigrants enrolled in it.
The organizations argue they first asked for the information via a Freedom of Information Act request in September, and that ICE and DHS did not respond.
"We're now at this new frontier of immigration enforcement, where digital surveillance plays a huge role and the kind of information that ICE is collecting and the net that they're casting is so large that the system is looking completely different," says Cinthya Rodriguez, an organizer for Mijente.
Sejal Zota, co-founder and legal director of Just Futures Law, says she's hoping the lawsuit will answer questions about the program and what she and others refer to as ICE's "digital prisons."
"It's very unclear to us what information ICE and BI are collecting on immigrants and how that information is being used or could be used," she says. "That lack of clarity raises alarms about people's privacy, future uses of this data and the reach of this surveillance dragnet."
Meanwhile, she says, the program is needlessly causing emotional harm to tens of thousands of people. Some people enrolled in the program, she says, are told their case manager may call them at any point on a given day, making them scared to leave home.
"Imagine an app, it can call you any time, while you're at work, while you're cleaning someone's house. It makes weird sounds. It draws people's attention," Zota says.
An ICE spokesperson declined to comment on the lawsuit, citing the agency's policy against commenting on pending litigation. But in a May 23 court filing, the agency asked a judge to dismiss the case, stating that the organizations' initial FOIA request did not sufficiently describe the records sought and that some information requested may be exempt from disclosure. Government attorneys also denied the lawsuit's allegations about the program and its impact.
ICE has said it's "committed to protecting privacy rights, and civil rights and liberties of all participants" in the program, and that a privacy analysis of the program was conducted and approved by DHS.
GEO Group declined to comment, referring questions to ICE. On its website, the company says that it complies with all federal privacy laws, that it does not conduct surveillance and that all data and records collected under its contract with ICE are the property of the US government.
The program has also come under fire from conservative critics, who say they're worried it gives would-be migrants a greater incentive to take their chances at the border.
"When you start handing out cell phones and say, 'We'll buzz you when we need to get in touch with you,' and then they let them go on their merry way into the interior of the United States, that's just going to encourage more and more people to come," says Chris Chmielenski, deputy director of NumbersUSA, an organization that advocates for less immigration.
Instead, Chmielenski says, anyone who crosses the border illegally should be immediately kicked out of the country.
"Then you don't have to deal with detention centers or alternatives to detention," he says.
During a recent hearing over the Department of Homeland Security's proposed 2023 budget, Rep. Jake LaTurner, R-Kansas, expressed his dismay over plans to grow the ATD program.
"Do you really believe that mass releasing those who illegally cross our borders does not send the wrong message to human smugglers, cartels and migrants?" he said. "Isn't this yet another message by this administration that the US will not enforce our immigration laws?"
"No, that is not the message at all," Homeland Security Secretary Alejandro Mayorkas responded. "The individuals are in immigration enforcement proceedings. And if their claims for relief under the laws of the United States are not successful, they are subject to removal. And the appearance rate of individuals on our alternatives to detention program have, in fact, increased."
LaTurner was unconvinced.
"This program, without question," he said, "is another message that the borders are wide open. Come on in."
The rapid growth of the program shouldn't just concern those with ties to the immigration system, says Kocher, who's also an assistant research professor at Syracuse.
"This technology is expansive," he says. "And it's not as if this is going to stop with immigrants either. It could be anyone. It could be having outstanding tickets in your city. It could be students on suspension from school. This technology could be increasingly used by your employer to track what you're doing at work."
Opponents of the way US immigration authorities use technology often point out that tools initially deployed as part of immigration enforcement, such as border surveillance, later can end up expanding into other communities and uses.
Kocher and other critics of the alternatives to detention program argue the resources used to fund it could be better spent elsewhere. This fiscal year's budget includes more than $440 million to fund the program. And officials are requesting $87 million more for next year.
Immigrant rights advocates argue that providing legal representation to those with cases in immigration court would do even more to guarantee that people keep showing up. Immigration restrictionists argue the resources would be better spent on detaining and deporting more migrants who are in the country illegally.
Neither of those options appears to be imminent. But the push to expand alternatives to detention remains a clear priority.
ICE recently renewed a contract with another private company to provide 10,000 cell phones outfitted with a facial recognition app for migrant check-ins.
For his part, Joel says he plans to keep using the government-issued phone to check in every week.
He says police persecution and surveillance pushed him to flee Cuba. And in the United States, he says he's doing everything he can to comply with authorities so he won't get sent back.
It's worth it, he says, for the chance to live in a country where there's more freedom.