Does anybody have any data on what percentage of people actually click on a Google Places / Google+ listing VS call the business direct from the SERPs?

[Dave_Eddy]

I've had a few SMB clients who have experienced drops in website traffic once their Google Places listing has gone live. It's hard for the average SMB to understand that this may not be a bad thing because they actually may be received more leads direct from the local SERPs. So while I can try to explain this to my clients, it'd be nice to have some broad data on how searchers interact with Google local listings. I'd love to learn what percentage of people call direct from the SERPs instead of clicking through to the business' website link.

Obviously, the percentages would vary across different verticals, different devices & depending on whether the search query was branded or non-branded. I'm after some rough average data, so if anyone could point me in the right direction, that'd be great!