Scott Goldman uses his mobile phone to signal friends and business contacts all by the world, from Britain to Australia. But the Southern California-based consultant doesn’t pay a dime in worldly tolls to his mobile-phone carrier, AT&T, the biggest in the U.S.

Instead, Goldman places the universal portion of the calls — roughly 100 minutes a month — through a service called Gorilla Mobile that relies on Internet-based technology to route wireless calls virtually toll-free. Goldman, a user of Apple’s iPhone, estimates that he saves hundreds of dollars a year with Gorilla’s service. He stands to cut wireless bills even more by signing up for another, iCall, that’s due for the iPhone in the coming weeks. The service will let him place low-priced calls from Wi-Fi hotspots — bypassing the AT&T network altogether. “Once I can construct calls using the Wi-Fi network, I will, in all likelihood, reduce the monthly minutes I have [with AT&T,],” Goldman says.

Gorilla, iCall, and a growing number of other services rely on what’s known as Voice by Web Protocol technology that delivers speech via the World Wide Web in much the same way as e-mail. VoIP calling is already raising a ruckus in telecommunications, putting pressure on the price of land-line calling and luring subscribers toward upstarts like Vonage and Comcast away from incumbents such as AT&T, and Verizon. Now, the technology threatens to erode sales for mobile-phone service providers too.

Jajah’s Growth Spurt

By 2011 the number of mobile VoIP users around the world may rise to 100 million from 7 million in 2007, according to ON World, a consulting firm based in San Diego. ON World estimates that in 2011, mobile VoIP voice services may generate $33.7 billion, up from $516 million in 2006, the most recent year for which the figure is available. whether that sounds…

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