NOGALES, Ariz. — A group of migrants was hustling north through the southern Arizona desert the other night when one of their cellphones vibrated with a text message. "Watch out," it warned. "Things are hot up ahead. Take cover in the bushes."

The message, signaling the presence of the Border Patrol, was sent by a smuggler watching the group's progress through binoculars from a hillside on the Mexican side of the border, members of the group said later. It was part of what border officials and immigrant activists say is an emerging trend in illegal border crossing — the use of what is being called the cybercoyote.

"I've crossed eight times and this is the first time they've directed me with my cellphone," said Sandra Silva, 30, a native of Guadalajara, Mexico, who was on her way to Phoenix. "It's like a guide through the desert."

Increased enforcement has made it difficult to sneak into the United States, officials say. And repeat offenders caught in the act are more often receiving stiff prison terms, making smugglers more cautious about risking arrest themselves.

Guides still accompany the bulk of the migrants crossing the border, activists and Border Patrol agents say. Those guides are in regular radio contact with confederates, who warn of trouble ahead. But the Border Patrol has been noticing cases of migrants crossing alone but in cell contact with guides, said Mario Escalante, a spokesman for the Tucson office of the Border Patrol.

Mobile phones are ubiquitous in Mexico; many migrants consider them essential when crossing, right up there with sturdy shoes and jugs of water.

Now, though, in addition to using cellphones to keep relatives up to date on their progress, some illegal immigrants rely on them to keep out of the reach of the authorities. Silva said her group had no coyote with them but had received directions by text.

The messages typically come during migrants' first hour or so of hiking north, those who had used the new system said. If they make it that far, the illegal immigrants then meet up with guides on the United States side, who help them trek further north to waiting vehicles.

Aiding the process are numerous spotters, who monitor the southern Arizona desert from lookout points and help steer the migrants, as well as drug shipments, away from the authorities.

Smugglers are constantly innovating to elude the authorities, veteran Border Patrol agents say. "They always come up with new, clever ways of trying to avoid us," said T.J. Bonner, past president of the National Border Patrol Council, the union for agents. "This way minimizes the risk to smugglers and you're using a technology that is relatively cheap."

Migrants like the group of 13 that set out for Arizona on Tuesday night found that the system has serious limitations.

The group was led to a remote stretch of border in Pima County by a guide, who had jotted down their cellphone numbers and instructed them on how to configure their phones so they would switch from Telcel, the dominant Mexican carrier, to AT&T. The migrants said the cost for crossing was anywhere from $2,000 to $3,500, with some of them paying ahead of time and others putting down a deposit and agreeing to pay the balance once they crossed.

A guide assisted them in climbing the border fence, somewhere a few miles west of Nogales, and then directed them north on a well-worn path toward Sierra Vista, Ariz., the migrants said. Periodically over the next hour, text messages would arrive directing their course.

"They tell you to turn right or left or to watch out for the 'perrera,'" Silva said, using Spanish slang for the Border Patrol vehicle used to transport detainees.

But shortly after the last text arrived warning them of trouble ahead, they were spotted by Border Patrol agents, who took them into custody and deported them.

"We felt all alone out there," said Maria Martinez, 51, who said she had been receiving the texts and whispering the directions to others in the group but would have preferred having a guide actually with them. "We felt like we were in God's hands."

Cellphone coverage is notoriously spotty along the border and Martinez's battery was nearly dead when she was interviewed a few hours after her detention, which she said made her feel vulnerable. "If you're without power or credit on your phone, you're dead," said Martinez, who is from outside Mexico City and was en route to Oregon, where she has relatives.

To reduce the number of fatalities among border crossers, a University of San Diego professor, Ricardo Dominguez, has been developing a cellphone application to help guide illegal immigrants to water stations and other points of safety.

His project outraged three Republican members of Congress, who wrote to university officials last year condemning the research and suggesting that he may be violating the law by encouraging illegal immigration.

Already, migrants who find themselves in trouble use their phones to call for help. In one case from last year, the Border Patrol's search-and-rescue team responded to a 911 call received from a mountainous area in southern California. From a helicopter, the authorities saw a faint light from a cellphone and they were able to reach the ailing migrant, who was suffering from hypothermia and unable to walk.

Similarly, on May 1, a woman who had been abandoned by her guide in the Arizona desert called 911 to report that she had lost track of her 9-year-old daughter. The Border Patrol first found the woman in a remote area of Pima County and then, six miles away, the girl.

Cellphones help not just migrants trying to slip across the border but those trying to stop them.

Border Patrol agents have complained that a lack of coverage complicates their ability to communicate. On some stretches where coverage is not a problem, the Border Patrol has urged residents to report suspicious activity via text message.

After a rancher was killed along the border in 2010 in a high-profile case that remains unsolved, Rep. Gabrielle Giffords pushed to improve cellphone coverage in the region. After Giffords, a Democrat, was shot on Jan. 8, a Republican colleague from Texas, Rep. Ted Poe, introduced legislation she had supported to use federal grants to help beef up communications along the border.

"It was very obvious to me during my recent visit to southern Arizona that there are too many areas where cellphones simply do not work," Poe said in a statement in March.