The pay-per-call advertising market could reach $4 billion by 2009 in the U.S., with a conservative scenario placing that figure at $1.4 billion, according to a Kelsey Group report issued Tuesday to clients, MediaPost reports. The latter forecast assumes that 5 percent of calls from local search and internet Yellow Pages are monetized via pay-per-call, whereas the best-case scenario assumes 15 percent. AOL is the only major portal to distribute pay-per-call ads.

According to the study, calls are more effective than clicks at driving consumers to small and medium enterprises. The upward trend will be driven in part by consumers seeking for local information online, as well as by click fraud - if it comes to be perceived as a major problem. The study notes that a hybrid approach could work well - offering pay-per-click keywords or ads along with a pay-per-call phone number.