Skype director of business development for hardware and mobility, Eric Lagier, said [inhibitors for true mobile VOIP] are expensive data fees for cellular networks and the lack of High Speed Uplink Packet Access (HSUPA).

Smaller vendors such as Jajah and Fring have stolen ground from the big players in the mobile VOIP space by requiring users to log-in to Web portals prior to calls, or requiring middleware installations on handsets, which is already commercially viable.

Carriers' large-scale transition to VOIP may not begin in earnest until 2010, though spending on gear for the new calling technology will roughly double over the next five years, according to research company Dell'Oro Group.

Network equipment providers sold more than US$2.4 billion worth of dedicated VOIP infrastructure products last year, a number that will grow to US$4.7 billion by 2010, according to Steve Raab, an analyst at Dell'Oro.

Dell'Oro counted softswitches (servers that switch packet-based calls), media gateways (devices that translate between packet and circuit-switched networks) and products that combine those elements, Raab said.