Web sites that once pinched service pennies by hiding their phone numbers on hard-to-find Web pages are increasingly doing the opposite -- flashing prominent "click to call" buttons on their Web sites, inviting shoppers to pick up the phone and call them.

The telephone is insinuating itself more deeply into Web commerce, thanks to the way Internet telephony is lowering calling costs at a time when Web merchants are realizing they can boost sales by injecting a human voice into the sale of big-ticket items.

Click-to-talk is popping up not only as a customer service tool on Web sites, but as a sales lead generator in online ads. Some systems require customers to dial a special toll-free number and reach a call center.

Others invite people to type in their phone numbers online, and the company will call them back within seconds.

A bunch of advertising and telecom companies are racing to supply these click-to-talk systems to the leaders of Web commerce.

Run a search for "plumber" and "Fairfax County" on America Online, for instance, and you may see an ad with a toll-free phone number and a "call locally, toll free" message beside it. The ads are provided by Ingenio Inc., an ad network that partnered with AOL when it launched its new Local Search service this year.

Merchants placing the ads get assigned a special toll-free number by Ingenio, which allows Ingenio to keep track of who is calling the advertiser and where the caller saw the ad online. Advertisers pay only when a customer calls that number. Hence Ingenio calls its service a "pay per call" ad network.

Amazon.com offers a slightly different version in the Yellow Pages service it recently released in trial form. It is powered by Reston-based eStara Inc., a rival of Ingenio's. Instead of displaying special phone numbers, eStara pops up a box and invites visitors to type in their phone numbers.

EStara's technology automatically dials the customer seconds later, then patches the call through to the merchant's call center or, in the case of a smaller company, its place of business.

It remains to be seen how many merchants will pay for telephone leads online.

The Kelsey Group, a consulting firm that monitors the local ad market, predicts pay-per-call advertising will grow to become a major industry, generating between $1.4 billion and $4 billion in ad revenue by 2009.

Kelsey issued a report this week saying that even though business models around this fledgling ad system remain shaky, it expects the technology to become ubiquitous in the local marketplace.

"Consumers are accustomed to using the phone to contact local businesses," the report said. "Local businesses are similarly used to receiving calls and closing leads over the phone."

The idea behind eStara and Ingenio is similar -- helping merchants get customers on the phone so they can do a better job of selling to them. In the past, many merchants resisted giving Web site visitors a number to call because handling phone calls costs so much more than taking automated purchase orders over the Web.

"We were one of those companies that used to think, 'Oh gosh, we just want people to deal with us online,' " said John Swigart, chief marketing officer for Esurance Inc., which sells auto insurance directly to consumers online. He laughs recalling how Esurance once operated: "We would even encourage people to go back to the Web site when they would call into our call center."

Esurance stopped its call center from routing callers back to the Web site several years ago and began featuring its toll-free line prominently. Last summer it went further and implemented eStara's click-to-call technology.

In addition to automatically connecting customers by phone to Esurance's customer reps, the technology lets reps see the same pages a caller is seeing online and allows them to push particular Web pages onto the customer's screen.

Swigart said significantly more site visitors called Esurance when they were shown a "click to talk" button inviting them to type in their phone numbers than when they were simply shown a toll-free number.

EStara's system is not cheap. The firm charges a set-up fee of $20,000 to $40,000 and a monthly usage fee ranging from $5,000 to $25,000 -- or higher depending on call volume. But 350 Web merchants -- including Continental Airlines, Sprint, Macy's and Dell Financial Services -- are using it to reach out to customers, especially those buying expensive items. Some merchants only offer the feature to repeat customers who register and sign into their sites.

EStara's technology also powers an ad service used by online directories, including Verizon's SuperPages. Advertisers pay Verizon a flat monthly rate for a bundle of services that include the eStara call button.

EStara's revenue has doubled and call volumes have quadrupled in the past 18 months, Marketing Director Ian Halpern said.

Ingenio's pay-per-call ad market is also growing fast.

Unlike eStara, which sells its technology primarily as a customer service tool, Ingenio is trying to create an ad service that mimics those run by Google and Yahoo.

At Google and Yahoo, advertisers pay for the text ads appearing alongside search results only when users click on links in the ads. In Ingenio's system, advertisers pay only when users call the phone numbers displayed in the ads.

Marc Barach, Ingenio's marketing director, said advertisers have been bidding up the per-call ad rates in the company's network. Like Google and Yahoo, Ingenio runs an online auction in which advertisers compete for more prominent placement of their ads by agreeing to pay more for each click or call. Ingenio charges a minimum of $2 per call, and advertisers have already bid the average up to $6 or $7, Barach said. That is higher than traditional search ads, which typically average less than $1 per click.

Search marketing firm Did-It, which helps businesses figure out how much to pay per for search ads, increasingly is helping companies evaluate pay-per-call ads. Did-it.com ran a test with a hotel client this year comparing results from traditional search ads with newer, phone-enabled ads. It found pay-per-call ads led to roughly twice as many sales as pay-per-click ads.

"In some other research we are doing, it's even more," said Kevin Lee, the company's chairman.

Leslie Walker's e-mail address is walkerl@washpost.com.