Pay per call advertising has been a part of the search landscape for a very short time, and yet its potential as a distribution channel for website publishers and advertisers is immense. Mediapost reports that pay per call advertising could swell to over $4 billion by 2009. Is this the result of a new distribution channel for ads? Or a tried and true methodology built around the original "yellow pages" model? Either way, the upside is huge. Especially if you consider the following:

• Click fraud becomes non-existent. Either you received the call or you didn't. Easy to dispute, easy to track.

• Cost benefits - Yellow page ads are NOT a pay per performance model. As more people adopt broadband networks and become more comfortable with finding local businesses on the web, pay per call distribution will grow.

• Targeting benefits - Vertical publishers will benefit immensely, as will the relevant advertisers.

A list of first-tier pay per call or "pay per lead" search engines:

• Miva.com - Advertiser places bid based on willingness to pay for a call. Formerly known as findwhat.com.

• Ingenio.com - The other of two major players in this space. Major distribution partner in AOL.

• Jambo.com - Launched after both Ingenio and Miva launched their paypercall services, but growing rapidly.