Google's patent...
"Ringback Advertising
Methods, systems, and apparatus, including computer program products, in which an indication of a telephone call being placed from a calling number is received, and a determination is made of an audio advertisement to play based on the calling number. The audio advertisement is played based on the determination."
http://www.engadget.com/2012/03/13/google-ringtone-advertising-patent
So when you make a phone call, based on the number you are calling, you'll get a certain advertisement.
Imagine you are using Google Voice, you have your contacts in Google, and those contacts probably also use Google services. So Google knows a lot of information about your contacts, they probably know their interests better than you. So maybe you call your friend he is a gamer and you'll get some sort of gaming related advertisement maybe.
But the main concern is those of us that have ported our mobile numbers into Google voice and have been enjoying this free service, well the free ride might be ending eventually as we might soon have to deal with advertising.
It's just a patent right now, so Google may never even use it, who knows.
