Virgin mobile announced free mobile calls, if you watch or listen to the advertisement before the call. (virgin free calls)
It looks like interesting concept and I am sure, that a lot of young people will like to use it. For mobile operators, it can be big a thread too, they are not used to sell advertisement space to third parties, plus revenues from such a channel is harder to predict than from calls. (We know, that people will call to each other in the future too)
But it definitely looks like good weapon againts Skype and free wi-fi calls.
In the Czech republic, one operator offered free SMS from internet, but 80 character long advertisement was included in the message. This service was not very popular, so I am looking forward to see, if free calls with advertisements will be something what customers wants and will accept.