VoodooVox, whose system for inserting audio advertising into phone calls could become the foundation for free cellular service, has landed $8.1 million in  capital from a stable of blue-chip venture capitalists, the company said on Tuesday.

The series D round was led by Softbank Capital with participation from prior investors Apax Partners, Village Ventures, and Steamboat Ventures, Disney’s venture capital arm.

As technology giants such as Apple, Google, and Microsoft extend their franchises to mobile devices, voice has become a new flashpoint in the battle for audiences and profit.

And the battle is clearly on: Apple has raised its flag on the mobile map with its new iPhone. Google has launched an experimental free telephone search service and is expected to roll out its own “Gphone” or a specification that would allow others to create one. In March, Microsoft, whose mobile operating system already resides on some phones, acquired voice search provider TellMe Networks in a deal estimated at $800 million.

Softbank Capital partner Steve Murray likened VoodooVox to online advertising networks such as DoubleClick, scooped up by Google for $3.1 billion in April.

“We look at it as another generation of the interactive ad networks,” he said. 

VoodooVox, which cut its teeth inserting advertising messages in the phone calls received by its more than 400 radio station clients, can insert the audio messages not only in standard landline or mobile calls, but also through sponsored calling cards and Web 2.0 voice widgets.

Mr. Murray, who joined the VoodooVox board of directors, said competition is scarce—for now. Startup Jingle offers advertising-supported directory assistance while Yahoo, Google, and Microsoft have not yet created advertising-supported voice services.

“It would be logical to assume that at some time they may,” he added.

In total, VoodooVox, based in North Adams, Massachusetts, has taken in about $14 million in venture funding.

VoodooVox Chief Executive J. Scott Hamilton said the company’s “publishers,” the clients into whose phone calls the ads are inserted, will rack up 300 million phone calls in September. He said the click-through rate—in which callers punch a key to get more information or conduct a transaction—is up to 12 percent.

Still, he said the company faces hurdles in persuading advertisers to spend a slice of their budget on the relatively new commercial form.

Among the advertisers already on board are Disney, MTV, and telephone calling card company IDT.

Eventually, Mr. Hamilton said, advertising will support mobile telephony itself, a view espoused last year by Google Chief Executive Eric Schmidt.

“Where we think the industry is going, we think someone is going to provide free cellular service,” he said. “That’s where I think the enormous opportunity is.”