My primary question is about tracking. How do I know how often a phone number on my site produces a phone call? Assuming a person receiving those phone calls knows that I can't track calls, why should he pay me in full? Granted, he'd have to pay something or risk me taking down the phone number. But it seems like I'd be setting myself up to get cheated out of an unknowable percentage of advertising revenue.
There may be a simple answer; but, intriguing as I find pay-per-call, I haven't researched it yet.