With call extensions, advertisers can either use a company business number or a Google forwarding phone number – Google will dynamically generate unique phone numbers per ad group and calls are routed to your business number.
Note: Click charges apply to visitors that click phone numbers using a mobile device. The calls will cost $1 USD per call for other types of calls to the Google numbers.
It’s worth noting there are several places in the Google interface advertisers can get information on incoming phone calls:
To see number of calls received, segment by “click type” at the ad group or keyword level.
For more detailed reports, go to the “dimensions” tab and select “view” then “call details”.