With call extensions, a customer who searches for Thai food on her mobile phone can see an ad for a Thai restaurant, along with a phone number, and make the call with one click. How is that priced?
The same as when someone clicks on an ad
Flat fee, based on the caller’s phone model
Negotiated in advance, with bulk discounts
By the minute, based on the length of the call
You’re charged as usual for clicks on your ad, as well as for certain interactions that extensions provide. For example, you’re charged for clicking on a download button (app extensions), on a call button (call extensions), on a directions icon (location extensions), and so on.
Read more here: https://support.google.com/Google Ads/answer/2375499?hl=en