It is 2am on Sunday and the phones never stop ringing at the Caracol radio station in northern Bogotá. The light banter that normally entertains listeners to the graveyard shift is missing. This weekly radio programme, Voices of Kidnapping, reaches out to the thousands of hostages held by illegal armed groups across Colombia.

"A warm welcome, wherever you are, to all hostages held in the mountains and jungles of Colombia," says Herbin Hoyos, the host and veteran war journalist.

Esperanza Moreno is one of the first callers to get through. Her son was kidnapped seven years ago by the leftwing guerrilla group the Revolutionary Armed Forces of Colombia (Farc). She does not know whether he is listening, or even if he is alive, but she regularly calls the programme. "I love you with all my heart. Keep strong. We haven't forgotten about you," she says.

She begs the guerrillas to release her son. "Please give him back to us," pleads Ms Moreno. "Give us some news, even just a letter." The Moreno family has received no news for two years. The uncertainty and desperation is evident in her wavering voice.

"One minute left," interjects the show's host. It is at this point that Ms Moreno, like most callers, breaks down. Her faltering voice gives way to silent tears. She simply ends: "Speak next week, I'm passing you on to daddy."

The Voices of Kidnapping programme is the only way for families to communicate with some of the 4,200 hostages in Colombia. For the next three hours, around 250 callers will send messages of comfort and hope. Families of foreign hostages held in Colombia call from abroad, leaving messages in foreign languages.

They also talk about daily life: how the kids are doing at school, what they did over the weekend. At first their messages are upbeat, but ultimately their composed voices give way to a tangible sense of profound sadness and hopelessness.

"Over the years, the programme has become a brutal record of people's lives," says Mr Hoyos.

The host understands the anguish that hostages experience. He himself was abducted by Farc 13 years ago. His time in captivity inspired him to launch the radio programme. "You only truly value your own liberty when it's suddenly snatched away from you," he reflects.

Kidnap victims do listen to the programme. Framed letters of gratitude sent by released hostages, once avid listeners of the show, hang on the studio walls. And the radio programme has saved lives. "Released hostages have told me that listening to the show was the only thing that stood between them and suicide. It gave them the strength to carry on living," says Mr Hoyos.

The programme aids the guerrillas too. After months of captivity, many hostages begin to lose the will to live, and refuse to eat. "A dead hostage is a useless one," says Mr Hoyos. "If it means keeping them alive, Farc allows them to listen to the show." As a result, improvised radios have become a vital part of guerrillas' kidnapping kit.

Since the launch of Voices of Kidnapping in 1991, more than 15,000 people have been kidnapped in Colombia, of whom around 1,900 have died in captivity. Traditionally, Farc has been responsible for the majority of kidnappings but lately common criminals are snatching a greater share of the profits, estimated to be worth more than £108m a year.

Paying a hefty ransom normally guarantees a hostage's release. But those kidnapped for political motives have little hope. They are used as bargaining chips in a war of attrition between the Colombian government and Farc. The government has repeatedly rejected Farc's offer to swap dozens of hostages, including US citizens and Ingrid Betancourt, a former presidential candidate, for scores of guerrilla members held in state prisons.

Erica Alberto is another regular caller to Voices of Kidnapping. As the wife of a politician she knows that the chances of her husband being released are slim. He was abducted three years ago when Farc soldiers stormed a municipal building.

Ms Alberto was pregnant and her husband has never seen their son, now almost three years old. But she gains solace from the hope that he may have heard their child growing up on the radio. At first, it started with gargled baby sounds but over the years the baby has begun to speak. "Say hi to daddy," she prompts their child. He mumbles cheerful greetings to his unknown father. "There are no words to describe the pain," says Ms Alberto as her three-minute allotted airtime comes to an end. "Despite the distance between us, we remain united."

At a glance

· Around 4,200 people are being held hostage in Colombia

· Kidnappings have fallen by 73% since Alvaro Uribe came to power in 2002

· Farc rebels use ransom money to fund its war with the government

· Last week Farc and Mr Uribe agreed to discuss an exchange of hostages for Farc prisoners

· Two British backpackers were kidnapped in September 2003. Matthew Scott escaped and was rescued after two weeks alone in the jungle; Mark Henderson was released after three months

· Farc is holding presidential candidate Ingrid Betancourt and running mate Clara Rojas. It is said Ms Rojas has a child with one of her captors

Katy Heslop