Google announced yesterday that they now support bid adjustments for click-to-call ads. So now you can bid more for click-to-call ads over other ads. Why would you do this? Google says phone calls "convert three times better than web clicks."
So now under your bid adjustments you can set the calls to a higher CPC based on expecting a higher conversion rate.
Google's help document explains:
You can increase your bid for mobile devices to show call interaction ads more frequently for mobile phone users. Interaction bid adjustments influence how often you show call extensions and call-only ads to users. The flexibility of adjusting your bids for certain interactions gives you more control over how often they appear with your ad and how potential customers connect with your business.
Will you be increasing your CPCs for calls?
Forum discussion at Google+.
