A patent application called “Ringback Advertising“ is assigned to Google, which seems to be Google’s plans to monetize Google Voice service by play ads while a call is dialing or placed on hold. It describes a system for delivering ads to any sort of phone system, including IP, cellular, or landline phones. The idea is to place software somewhere within the flow of telephony data that can identify when a given call is not active, then request audio ads for delivery during that time.
The patent identifies three specific times that ads might be appropriate: when a caller is dialing, when the call is suspended via call waiting, and when the call is placed on hold. The ads can also be targeted based on information about the person doing the dialing, such as geographic location or interests obtained from a user-provided profile.[via ars, thanks!]