Google has updated a patent application filed last year for "ringback advertising," a form of audio advertising that appears to be intended to work with the company's Google Voice service.

Audio ads, the patent application says, may unnecessarily annoy consumers. But providing audio ads during a time otherwise used to play a ringback tone may allow advertising without annoyance "because the consumer would have to otherwise listen to some other form of audio."

Google Voice provides a single phone number that routes incoming calls to as many as six of the user's phones at once, along with sophisticated voicemail service. It also serves as a call bridge: When a user enters a phone number into the Google Voice browser form (or one of the new ">Google Voice mobile apps), the service dials that number and one of the user's phones, then connects the two parties.

Although the patent application makes no mention of Google Voice -- the service wasn't public when the application was first filed and was called Grand Central at the time -- it explains how audio ads can be used in conjunction with a "call-back engine," which accurately describes the function of Google Voice.

It's during this "call-back" connection process that Google anticipates replacing ringback tones and with audio ads.

Google Voice product manager Vincent Paquet has acknowledged that Google is looking into advertising as a way to monetize Google Voice.

The patent application describes extending Google's ad auction process to audio ads and uses the term "cost-per-call" rather than "cost-per-click."

It also describes the audio version of the "click-through," the billable event in cost-per-click advertising, as a "listen-through."

"A listen-through can occur, for example, when a user of a user device, listens to some or all of an audio advertisement provided by the advertising management system," the patent application states.

Google, however, isn't the first company to contemplate such a system. A 2007 patent application, "Online Production and Media Coordination Portal/System for Telephone Ringback Messages and Digital Media Content," appears to cover similar concepts, including ringback advertising.

Whether there's any overlap between this patent application and Google's will be a matter for the U.S. Patent Office and patent attorneys to sort out.

Registration is now open for the leading enterprise communications event, VoiceCon. It happens in San Francisco, Nov. 2-5. Find out more and register.