Google apparently has a patent application out for advertising on mobile phones, according to William Slawski's SEO (Search Engine Optimization) by the Sea blog.
The patent application abstract says: "At least some ads may include call-on-select functionality," which would automatically dial the advertiser's phone number when the user clicks on a button.
Slawski concludes: "If the search engine is going to serve ads on mobile devices, I do like the idea that they will take into account small displays and slower connection speeds. I suspect that there's no way to avoid ads on a smart phone."
Already Google has been testing a new click-to-call service that lets people speak with some advertisers on Google's search results page. A Web surfer can click on a phone icon adjacent to an ad, enter his or her phone number and click a "connect for free" button and the phone will ring with the advertiser on the other end.