Some of the families have received calls from those numbers, from people claiming to be holding their husbands or sons and asking for ransom to get them home.

Sometimes the captives have phoned themselves in all-too-brief calls, pleading for their families to heed the demands. The families call back but either nobody answers or, if they do, the phone goes dead.

In recent weeks, as reports of mass graves of migrants emerged from Malaysia and Thailand and images of hundreds of starving people abandoned at sea spread around the world, the calls to families in Teknaf have become more persistent.

Khaleda Banu, from a dirt-poor fishing settlement, said a man phoned her four days ago saying her 19-year-old son, missing since November, was alive. She could have him back if she handed over 30,000 taka ($385) as part payment.

But she had already paid that amount months ago when the first call came and nothing had happened. The man hung up and wouldn't answer her calls and now the mother, surrounded by neighbors, is sick with worry.

"My son just slipped away. I was sleeping when he left," she said. She found out later a man claiming to be a recruiter had persuaded him that his carpentry skills would earn him 10 times more money in Malaysia than at home.

She filed a case against the agent who has since been arrested. But the agent can't help her find her son.

Thousands of people, mostly Rohingya Muslims fleeing persecution in Buddhist-majority Myanmar, but also Bangladeshis have been trying to slip into Thailand, Malaysia and Indonesia for years through people-smuggling networks.

More than 4,000 migrants have landed in Indonesia, Malaysia, Thailand, Myanmar and Bangladesh in recent weeks since Thailand launched a crackdown on people-smuggling gangs. Around 2,000 may still be adrift in boats on the Andaman Sea and Bay of Bengal, the United Nations said.

The journey for a large number begins in the coastal town of Teknaf bounded on one side by Myanmar from where the Rohingya cross over, and the Bay of Bengal on the other. A hub of the narcotics and smuggling trade from Myanmar, Teknaf is the shortest route to Thailand and Malaysia.

"MALAYSIA AIRPORTS"

The traffickers use a route along the edges of Bangladeshi and Myanmar's waters, darting across the border back and forth to dodge security forces of the two countries, taking advantage of the lack of joint patrolling, officials say.

Some 3,000 fishing boats, most of them long-bowed, ply the waters that are used by traffickers to move their people cargoes. There are 60 jump-off points along a 100-km (60-mile)stretch of powder-white sand, reputed to be the world's longest unbroken beach. Locals refer to the departure jetties as "Malaysia airports".

Some families said that the traffickers had lured their loved ones by not asking for money at the beginning of the voyage to Southeast Asia.

To many in Bangladesh, which exports tens of thousands of workers each year across the world, that seemed too good an offer to turn down, having seen friends and relatives had over huge amounts in previous years to go abroad.

Mohammad Hassan said agents told his son-in-law, who earned a living making fishing nets, that he could pay for the journey after he had found work in Malaysia.

Then, months after he went away, the call came: the agents demanded that Hassan pay a ransom of 220,000 taka ($2,830) to get him freed from a Thai camp where he was being held.

"I told the man I am a daily wage laborer. Where can I arrange that money? I don't have things to sell."

A week ago, Hassan said, tears welling up, he received a call saying his son-law-law and a father of three had been killed and his body buried on a hillside in Thailand.

"This has become an extortion racket. They are preying on gullible people," said Mohammad Abuzar Al Zahid, commanding officer of the Border Guard Bangladesh responsible for the defense of the Teknaf area.

"These brokers have spread everywhere, in the villages, in towns. They are like veins in your body."

Authorities say the traffickers are part of a network that for years has preyed on the Rohingya, a largely stateless minority in Myanmar.

The gangs promise to deliver the refugees to Muslim-majority Malaysia and in recent years targeted Bangladeshis too. Typically there are about 500 to 600 Rohingya on a boat along with some 200-300 Bangladesh nationals.

FACELESS MIDDLEMEN

Sometimes the gang members don't even bother taking their victims to the sea, keeping them confined below decks in a fishing boat moored in the placid waters of the Naf River and making ransom calls to their families, Zahid said.

From the man who makes the first contact with a potential migrant to the international agents into whose hands he is finally passed, its a web of faceless middlemen who don't know each other and cannot ensure their security, said Zahid based on the interrogation reports of traffickers who had been caught.

"There is a whole support group, the boatmen who take these victims to the cargo ships, the people who sell foodstuffs for the journey and then the people who collect money."

A newspaper quoted the owner of a trawler as saying that his entire 17-member crew, supposed to have gone fishing, had sold the vessel to traffickers in return for getting them into Malaysia.

In Teknaf, there are shops selling large water containers, sacks of puffed and flattened rice, fruit, biscuits and drinks for the job seekers embarking on the journey.

"The administration has only now woken up when graves turned up in Thailand and people are floating in the sea," said former MP from the area, Mohammad Ali. "Law enforcement has been negligible."

But Bangladeshi Prime Minister Sheikh Hasina said migrants trying to leave the country illegally were partly to blame for their predicament.

"They are tainting the image of the country in the international arena and putting their life into danger," state media quoted her last week as telling government officials, calling the migrants "mentally sick".

"There is sufficient work for them. Still they are leaving the country in such disastrous ways."