William says: "The recent police action has made me very careful about the way I work."

He is referring to the online vice ring bust last December.

About 60 men were arrested and later released on police bail. They are accused of having had sex with a "social escort".

Prostitution is legal here, but it is a crime to solicit by approaching a customer or promoting the trade. This also means it is an offence to be a pimp.

It is also illegal to have paid sex with a girl under 18. Anyone who does so may be jailed for up to seven years, fined up to $10,000, or both.

Anyone who makes or organises travel arrangements for another person to engage in paid sex with someone under 18 outside Singapore is also committing a crime.

In William's case, he's careful not to include underage girls in his books, though he knows that the demand for them is "high".

But having young girls in his stable is not without its headaches.

"Sometimes, these girls are busy with school, so I have to know who is available and make the recommendations to the customers," he says.

It is understood these girls are Singaporeans and studying in tertiary institutions.

"But the truth is these young girls don't stay in the business for very long and only do it when they need the money," he adds.

The price for their services ranges from $200 to $800 and is set by the girls themselves.

He also claims that the girls in his stable are all acquaintances introduced to him "by friends because they want to earn extra cash".

These friends, he claims, would forward them his contacts. Most of his dealings with these girls are via SMS, he adds.

Pimps, however, are known to recruit the young women through advertisements online.

But while some escorts prefer to have someone like William making their arrangements, some - like Celine (not her real name) - prefer to work alone.

That way, she's less likely to be implicated in any vice busts.

"Working alone also give me the chance to pick and choose my own clients," says the attractive 19-year-old, who is charming and exudes a confidence which attracts a lot of male attention.

She is currently unemployed - she dropped out of school last year - and says she meets "five or six customers a week" when in need of cash, charging them $250 per hour each time.

Celine is confident that her skills, honed from two years in prostitution, have kept her regulars coming back for more.

"I have regular clients who meet me often and they said I'm good," she says matter-of-factly.

She got her start in the business when a man she met on an Internet chat line offered her $300 for sex.

Celine, then a 17-year-old first-year polytechnic student, says: "Whenever I chatted with these men online, we flirted and talked about sex just for fun.

"Then the guy made the cash offer... I needed the money for expenses, rather than work a part-time job, I thought I could make money this way."

She would meet her clients in the afternoons, sometimes after school.

"In the beginning, it was by word of mouth, I would get new clients based on recommendations but these days, I advertise on forums and certain websites," Celine explains.

But the heat is on after the recent arrests.

The men nabbed in December may soon have to appear in court, and last Friday in a separate case, nine men were charged in court for commercial sex with three Vietnamese girls below 18.

William says he is more selective with his clients these days. He also uses a separate pre-paid phone card when dealing with customers.

Typically, he will SMS his customers to give them the time and venue to meet the girls.

He admits it's "dangerous" to have his mobile phone number out in the open.

So as a precaution, he changes the number regularly.

William says: "I also screen the people who contact me for services, to make sure they are reliable."