LONDON -- About 500 yards from the Houses of Parliament is one of Britain's distinctive red telephone booths. Its exterior is a British icon often photographed by tourists. Its interior is a billboard for London's prostitution trade.

One recent afternoon, three colorful cards, with pictures of scantily clad women on them, were posted in the booth. One read: "New to London, Alina." It gave a telephone number and added, "A place where your dream comes true." Another was more direct: "Mature Suzi, Busty 46H."

The cards are at the center of a war between London officials and phone companies over how to crack down on vice in the city. Kit Malthouse, deputy leader of Westminster City Council, the government body that runs the schools and cleans the streets in the neighborhood, wants phone companies to strangle the prostitution trade by refusing to put calls through to the numbers on the cards.

"You have got to put your foot on the dustbin lid," says Mr. Malthouse, a burly Conservative member of the council.

The United Kingdom's landline phone companies have gone along. But most cellphone companies have balked -- even after the council staff handed out 20,000 mock prostitute cards with the names and phone numbers of cellphone chief executives on them.

The cellphone companies claim it isn't their job to interfere with a customer's service. "We are not content to play the role of moral arbiter," Phil Kirby, head of regulation at the U.K. arm of Vodafone Group PLC, the world's largest cellphone company, wrote in a January e-mail to Mr. Malthouse.

Mr. Kirby said Vodafone doesn't want to set any precedents for helping the government act against customers accused of being a social nuisance. Vodafone and the others add that they don't want to be judges of wrongdoing.

T-Mobile, which is owned by Deutsche Telekom AG, refused to block calls, too. It doesn't want to be sued by customers. It added, in its own e-mail to the council, that blocking calls could jeopardize the safety of prostitutes, which is an argument also made by advocates for prostitutes' rights. "These women have families and people that need to reach them," says Hilary Kinnell, a spokeswoman for the U.K. Network of Sex Work Projects.

Niki Adams, a spokeswoman for the English Collective of Prostitutes, says that women who are unable to advertise in phone booths may be forced to walk the streets, which is more dangerous than operating from an apartment that has a telephone. Ms. Adams says her group will sue if all the telephone companies agree to block calls.

Mr. Malthouse, who is 38 years old, says a crackdown is crucial because the world's oldest profession has been booming in London since organized crime gangs coerced young women from Eastern Europe and Russia into the London sex trade after the fall of the Berlin Wall. The Poppy Project, a charity that helps women escape the sex trade, did a survey in 2003 finding that there were as many as 5,900 prostitutes in London, working out of 730 apartments, parlors and saunas.

Prostitution itself is not illegal here, though soliciting for business on the street and keeping a brothel are. The trade has relied heavily on posting cards in phone booths to advertise to tourists and business travelers.

BT Group PLC was the first to try to rub out the cards. The phone company owns and cleans most of the telephone booths in London. In 1996, BT began blocking calls to numbers on the cards and got other phone companies to go along. But the London Committee of Call Girls, a defunct group that represented prostitutes, complained to the government's Office of Fair Trading that the telephone companies were colluding to restrain trade.

BT was forced to put its plan on hold, while it wrote to other phone companies to make it clear that it wasn't trying to establish a formal agreement on call barring, says Les King, a BT spokesman. Confident that it had done enough to satisfy the Office of Fair Trading, BT in 1997 began issuing warnings to people posting unauthorized advertising in its phone booths. If the ads persist, it stops incoming calls to the numbers in the ads. In the two years ended last July, BT says, it sent out 444 warning letters and cut off calls to 84 phone numbers.

That hasn't helped much. Prostitutes and pimps simply switched to other service providers, including T-Mobile, Telewest Global Inc., and Vodafone, BT says. And the cards continued to flood in. "We clean every box on busy sites, five or six days a week," says BT's Mr. King. "We take out more than a million cards . . . each year."

It's not clear how big an impact call blocking would have on the prostitution business. A woman who called herself Annabella answered a reporter's call to the number on a card advertising a "Busty Black Beauty." Annabella said she had several prostitutes working for her and that she would find ways around any call blocking. "We have got plenty of places to advertise," she added. Ads touting the services of prostitutes appear on the Internet, in magazine classifieds and in the windows of some shops.

Mr. Malthouse took up the cause five years ago. The owner of a financial firm who has run unsuccessfully for Parliament, he isn't afraid of confrontation. He was galvanized to tackle the phone-booth issue after parents complained that their children were collecting the prostitutes' cards like Pokemon or soccer cards. He asked government telephone regulators to enforce call blocking. They told him to get Parliament to pass a law against posting prostitute cards in phone booths. Such a law was then passed in 2001.

Since then, the London police have engaged in periodic crackdowns. In one code-named Playa, in the summer of 2003, the police and BT's team of in-house detectives traced the telephone numbers on cards to several addresses in central London. They raided those addresses, seized 350,000 cards and arrested 23 people.

But the police acknowledge they can't monitor phone booths all the time and that catching people dropping off the cards is tough. "It is not something that you can put resources into every day," says a spokeswoman for Westminster's police force.

Over the years, Mr. Malthouse tried unsuccessfully to persuade the cellphone companies to block calls. Last summer, he decided to make a nuisance of himself. He designed and had printed up 20,000 mock prostitute cards with pictures of women -- and the names, numbers and business addresses of the CEOs of the biggest cellphone companies. The cards urged people to write to the CEOs and tell them to block calls to prostitutes' numbers.

Council staff handed out the mock cards on Oxford Street, London's busiest shopping street. Carrying 6-by-10-foot placards designed to look like giant prostitute cards, they also picketed the shops of the cellphone companies.

Simon Duffy, the CEO of cable company NTL Inc., was one victim of the protest. He heard from his doctor, who had seen his name and phone number in a British tabloid. And he got several letters. Oddly, Mr. Duffy says, he got no phone calls. NTL, which offers phone service to its cable customers, now blocks calls to numbers on the cards, Mr. Duffy says. Telewest, another cable company, agreed to bar calls, too.

In October, Mr. Malthouse invited the major cellphone companies to meet in his office and made his case for blocking prostitutes' incoming calls. He asked for a six-month trial.

The cellphone companies agreed to mull it over. But in late January, both Vodafone and T-Mobile told the council they won't block calls. T- Mobile says that instead it is waiting for the government to announce a new plan for tackling prostitution. Mr. Malthouse promises more publicity stunts targeting the cellphone companies.