I tried pay-per-call before. Never got a valid lead from it.
The only calls I got were from competitors and people who misdialed and got to me, or found my # listed somewhere.
Ingenio was recycling old 800#'s and due to that they would be old support #'s for defunct products, companies etc.
We'd get calls like "Do you have part #888444 I need it for my tractor and it says to call this phone #"
Keep in mind I'm a web design and internet marketing company...
yeah... so pay-per-call great in concept is just not working...
If Google wants to improve on this idea they could offer to remotely host a contact form and give you a trackable 800#, then you make a clone of your site with no email contact, only this google form and google phone #.
You would then, in turn, pay only per call or per lead... I think they could make that work!