AT&T has agreed to acquire privately held Ingenio, a provider of so-called “pay-per-call” technology, for an undisclosed amount.

Ingenio lets businesses bill clients when they call for information, instruction or other educational purposes. It also gives businesses a way to advertise online, letting them provide their phone numbers within online ads.

When customers call the number in the ad, Ingenio tracks the frequency with which the numbers being called (it can do so, because it provides unique phone numbers for this), and charges a fee accordingly.

San Francisco’s Ingenio biggest backer was Benchmark Capital.

While the amount of the transaction wasn’t disclosed, Benchmark’s partner Kevin Harvey said the firm made good money on the deal.

"We made out well," Harvey told VentureWire. Harvey would not provide more details on the deal and would not speculate how other investors in the eight-year-old advertising-technology company might have fared.

"But the bigger (Ingenio) gets, the revenue scales independently of the expense line," Harvey said. "The cost of running the company is minimal."

Harvey has already had two big hits this year. He was the lead partner on Tellme, another voice company, bought by Microsoft, for about $800 million and Zimbra, the open source messaging company bought by Yahoo for $350 million.

A fourth company of his, MySQL, had been talking about going public, possibly by the end of this year. If it does, this would make it one of the most prolific years on record, among Silicon Valley VCs.

The Ingenio transaction is expected to close in early January 2008.

AT&T said it plans to integrate Ingenio’s features into its directory service and local search advertising portfolio, including the Network, AT&T Real Yellow Pages and 1-800-Yellow Pages.