Why No Major Ad-Supported Telephony Services?

I may have missed a trick here, but I'm not aware of any services that offer ad-supported telephony services - the only one I could think of that was remotely similar was JaJah. But this to me is an "old hat" version of what I'm thinking of.

With JaJah it uses VOIP technology and a web interface to connect people at a small cost but show banner ads on those pages at $2CPM.

They may have a million users and doing very well but why can't they actually insert 5 second adverts prior to connection - just like those adverts at the beginning of podcasts? With Google going in to radio advertising surely it's the next logical step?

It may not work for national calls as the costs are minimal and most people get free calls on their mobile. But what what about those with families abroad. With the barriers in Europe coming down every day and the free movement of labour surely there's a huge market for this sort of thing?

I'd be grateful if anyone knew of any company offering this type of service!