Large companies are leading the way in adopting VoIP adoption, adding it to their corporate phone options as their traditional PBXs age.

That's according to a new survey from Infonetics Research, which found that 36 percent of large US corporations are already using VoIP equipment and VoIP services.

Only 23 percent of mid-size and 14 percent of small businesses have adopted VoIP gear and services but Infonetics said this percentage is set to triple by 2010.

The main reasons businesses give for deploying VoIP include integrating phone systems across different sites and converging voice and data networks. But they also say they hope to save money on the cost of operating their phone networks, in part by paying less for long-distance calls.

Infonetics has conducted a similar survey for the past three years and they show a steady rise in the use of VoIP that will continue for the next three years without a dramatic up-tick in the adoption rate, according to the author of the study, Mattias Machowinski.

He says the steady increase is due to businesses turning to VoIP as their traditional phone systems reach the end of their life or as voice service contracts expire. At the projected rate of adoption, a half to two-thirds of large businesses will use VoIP by 2010, he said.

Among the businesses surveyed, the most often used IP PBX vendors were Cisco, Avaya and Nortel, the study says.

The study compared the amount organisations spent on VoIP services in 2005 and compared that figure to projections of what they will spend in 2007. The amount spent on hosted VoIP services jumped from US$47,667 to $63,799; the amount spent on managed PBX services jumped from $10,865 to $28,367.

The study also finds that the percentage of users accessing VoIP via Wi-Fi grows from 5 percent in 2006 to 20 percent in 2008.

The study was the result of interviews with 240 businesses using VoIP now or that will by 2007, as well as the results of 450 shorter interviews to determine VoIP adoption rates.