I have seen a few pay per call services mainly in the real estate industry and those have been very effective when the numbers were being featured on some good directories (real estate or home related ones).
While any big, medium or small business can have a website and use adwords. The Google Click-to-Call is very bad news for the medium and small companies as we do not have enough resources to have a proper call-centre to deal with all the calls. By introducing this tool Google is giving Big Companies a competitive advantage over the smalls one.
I guess for Google is easier to sell this product to multinationals rather than adwords itself like it is now.