Pay-per-call has been one of the trends in online marketing recently. In fact, pay-per-performance advertising, overall, is gaining ground as more and more businesses discover the high value proposition it provides. Different search engines are incorporating this type of marketing, including AOL search, InfoSpace and Marchex, who have a combined total of 1.3 billion searches a month.

http://www.practicalecommerce.com/articles/326-Pay-Per-Call-A-Smarter-Way-to-Advertise