KANSAS CITY, Mo. — Spam has long been clogging computers. Now, it's showing up on cell phones.

Spam has gone mobile. Increasingly, consumers are answering cell phone calls from telemarketers peddling time-share resorts, bogus lotteries or even porn.

And unlike computer e-mail spam, unsolicited text messages on a cell phone can be costly — 10 cents or more per message.

The problem is so new that federal agencies are unsure how to combat it. But they say that with millions of new cell phone users and billions of text messages being tapped out monthly, the practice is growing.

San Francisco-based Ferris Research, which tracks consumer messaging services, predicts that U.S. cell phone users will get 1.1 billion spam messages in 2007, compared with 800 million last year.

"As more and more people get cell phones, they are dealing with all sorts of products, services and consequences," said Lisa Hone, an attorney with the Federal Trade Commission's consumer protection bureau.

Wireless officials say they have learned from the experience of the computer industry and are taking aim at mobile spam. But it's a moving target.

"It is a constant and ever-changing threat," said Joe Farren, a spokesman for CTIA-The Wireless Association, a trade group. "As we develop new filters and firewalls, the spammers seek to develop new strategies as well. So it is an arms race of sorts."

Illegal operators often use automatic dialers. Spammers sitting at a computer can zap millions of calls addressed to random cell phone numbers. At the same time, cell phone numbers are illegally bought and sold on Internet sites that are started as fast as they are shut down.

In April, Verizon Wireless sued telemarketers it said had inundated the company with 12,022,411 unsolicited commercial text messages. Verizon said in its lawsuit that it was able to block all but 4,618. The barrage not only hit customers with "unwanted charges," but also clogged Verizon's networks, requiring the carrier to "dedicate equipment, software and personnel" to filter out the messages.

Spokeswoman Debra Lewis said the company was using everything in its arsenal to stop the spammers.

"Some still goes through," Lewis said. "People aren't expecting it because a cell phone is a private number."

Indeed, the expectation of privacy — cell phone numbers are not part of any directory — is one reason behind the explosion in cell phones.

Today, more than 233 million cell phones are in use, more than double the number in 2000. Text messaging is the fastest-growing feature. Since last year, Verizon users have more than doubled their text messaging, from 12 billion to 28.4 billion.

Among people 18 to 26 years old, the preference for text messaging has made e-mail the equivalent of snail mail. Text messaging is used to play games, vote on TV reality show contestants, register political preferences and check news headlines, weather and movie times.

But it is also used by legitimate businesses to reach subscribers and by first-responders to communicate in emergencies, Sprint spokeswoman Roni Singleton said.

"It's a service and a convenience and in some cases a vital communication tool," Singleton said. "That is why it is so critical to make this a matter of priority."

Sprint sleuths for dishonest marketers and helps find ways for subscribers to block unwanted message senders, she said.

The phenomenon of cell phone spam had not even been contemplated when the federal do-not-call laws were passed to combat unwanted telemarketing, and regulatory agencies have had to scramble to protect consumers.

In 2004, the Federal Communications Commission voted to ban all unauthorized text messages to mobile phones and pagers. The FCC also now applies the Telephone Consumer Protection Act — which prohibits autodialed and prerecorded calls to cell phones — to text messages. FCC guidelines now say text-based telemarketing solicitations sent to cell phones are covered by the national do-not-call rule. Some states are also trying to erect regulatory barriers.

But experts say some legitimate businesses do not want too many hurdles in the way of legal cashing in on the cell phone advertising market.

Google Inc., Microsoft Corp., Yahoo Inc. and AOL LLC are a few of the major Internet companies searching for ways to reach cell phone users with mobile ads. Using Global Positioning System technology, companies could deliver pop-up ads to cell phone users who are within blocks of a McDonald's restaurant, for example.

"It's too lucrative an opportunity," said Allan Keiter, founder and president of MyRatePlan, a Web site that monitors cell phone carriers.

Pop-up ads would be delivered only with a cell phone user's consent. However, Keiter predicted that consumers could be enticed with the promise of cheaper cell phone service, subsidized by commercials.