Greg Sterling, an analyst with the Kelsey Group, said that growth could be even faster, if portals aggressively sell the ads. "It's a mirror of online advertising in general," he said. "If you've got people pushing the product to market, it's going to grow much faster than if it's a purely self-service."

Google late last year began testing a click-to-call service; for that feature, the search giant displays an icon of a phone, then calls users who click on the icon and connects them with marketers.

Some marketing executives are eager to test the service, said Bill Leake, CEO of Austin, Tex.-based agency Leads Customers Growth. "Really, the only limitation at this point is how rapidly the portals roll out pay-per-call," he said.

He added that one of the firm's clients has increased its budget for click-to-call tenfold since April, when the client started running pay-per-call ads through Ingenio in April 2005. Ingenio currently has a deal with AOL, which is the only major Internet portal to carry a non-beta pay-per-click ad product.

The service is especially appealing to small, local businesses that don't have a strong Web presence, Leake said. "It's particularly exciting for the small businesses that don't have Web sites [that are] optimized for collecting leads," he said. "I could be a local plumber or lawyer who could clean up with pay-per-call, even if my Web site would send people screaming to the mountains if they saw it."

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