AT&T plans to buy online advertising company Ingenio for an undisclosed sum, in a move that puts the telephone company in competition with Google and other online advertisement leaders.

Ingenio's technology measures the effectiveness of advertisements by tracking phone calls made to businesses based on phone numbers used in ads. The ads can be displayed online, on mobile phone Web sites, and in print. The technology provisions the phone numbers and tracks the calls. Advertisers pay based on the volume of phone calls generated by the ads.

AT&T will integrate the technology into its yellowpages.com Web site. The operator said the technology would allow it to take advantage of the trend toward performance-based advertising.

Ingenio pushes the fact that advertisers don't need their own Web sites to use the service. The ads appear across Ingenio's network and point viewers to the phone number, not necessarily to a Web page.

AOL uses Ingenio's technology for pay-per-call ads, and MSN uses it for ads displayed to mobile phone users.

AT&T said it plans to keep Ingenio's management team. The company's Web site says it has 120 employees. AT&T expects the deal to close in January.

The operators of online advertising platforms, including Google and Yahoo, are increasingly looking for ways to display more targeted ads and improve the usage of ads. For example, some companies are experimenting with advertisements that let mobile users click on the ad and automatically call the advertiser.