In PPV no one can defraud you with fake clicks. You are not paying for clicks. And generating thousands of impressions for fraud purposes is less trivial task than just a few clicks here and there.
I 100% get what you're saying and see there is less risk - however, I'm just not buying mobile will work for online->offline phone calls after our massive failures (yes - we tried more than once). Are there any case studies out there of mobile campaigns bringing in real calls?