By Claire Marshall BBC correspondent in Bogota

"What you can see on that screen are two high-risk clients moving about the city. You can see the route they're on - one has just turned off Avenida 30, and the other is on Avenida 81," says security company boss Bill Nixon, standing in the operations room of his Bogota headquarters.

"They are in armoured cars which are satellite-tracked. Travelling covertly behind them on motorbikes are three-man response teams ready to act if there's an emergency. Basically we are monitoring them 24 hours a day."

To be liberated is the most extraordinary thing that can happen to anyone - it's like a birth of consciousness, you are being born again Jose Amiro Martinez Freed hostage

Three men sit at the control desk at security company Honor and Laurel, efficiently handling a constant stream of phone calls.

Radios crackle, and at various intervals, there are strange beeps. These are alarms triggered by some of the 6,000 clients being tracked from this room.

They have either used a recognised code-word on the telephone, or pressed a panic button. Each of them is paying Mr Nixon a minimum of $750 a day for protection here in Colombia - the kidnap capital of the world.

Lucrative business

All of the main irregular groups fighting Colombia's civil war are involved in kidnapping.

About 3,000 people are taken each year. Sometimes it can be political, but most often it is for money.

For international companies, to guarantee security on a simple site visit can become a military operation.

Some kidnap victims return, many do not

"People who are in the oil sector, for example, by the very nature of the business have to go in to red zones. Before they go in we would have negotiations with the military - they would put a sparse cover across the area," says Mr Nixon.

"We would then go in with a special team and they check routes and hotels and technically sweep the area for communications. Once we have secured the area we either then drive them in an armoured vehicle or fly them in a helicopter, so the route in and out is very quick."

President Alvaro Uribe's hard-line policies are getting results.

In office for little over a year, he has introduced strict new anti-kidnapping laws - and backed them up with force.

He is feared by the guerrillas. According to government figures, the number of people being snatched is down by more than 30% this year.

Not a word

That is not much comfort for the estimated 2,500 to 4,000 people currently being held in Colombia.

To disappear and be held hostage is such a common occurrence that unless you are well-known it would barely make few lines in a local paper.

FARC rebels have kidnapped thousands of people

However there is one radio programme which tries to let them know that they've not been forgotten.

On a Saturday night, Herbin Hoyos introduces "The Voices of the Kidnapped". It is essentially a weekly phone-in programme for friends and relatives to send out messages to their loved ones.

The lines are constantly jammed. The messages are heart-wrenching. One woman tries to keep her voice steady as she says: "My love, please don't forget all the reasons you've got to keep fighting... remember your family and our baby. This should make you strong and give you hope that you will be found".

Another wife has been calling in to the programme for five years. She has no way of knowing if her husband is dead or alive.

Tonight there is a special guest. Jose Amiro Martinez was recently released after being held for more than a year.

His family paid a ransom for him to be set free. It is an emotional moment as he comes in to the studio.

He hugs Herbin and smiles at the others - finally putting faces to the voices which became so familiar to him during his time in captivity.

He made himself a pledge to come and speak on the programme if he was released.

He tries to give the kidnap victims courage, saying just be patient - you are not forgotten.

"To be liberated is the most extraordinary thing that can happen to anyone. It's like a birth of consciousness - you are being born again".