OEM (Handset manufacturers) can monetize the real estate they have on the phone cover. Currently it's blank and can be imprinted with advertisers logo on the back. Such handset can be subsidized than the one with out an ads on it. The beauty of this is that when people talk, the cover of the phone gets displayed to other people. Not to dilute the value of such ads, there shouldn't be more than 2 ads on the cover.
It can be also be implemented by small business owners who are in business of selling the handsets. But if that's simple why not anyone doing it?