Google will begin charging $1.00 for calls completed using the unique number it provisions on AdWords ads, but solely when the call originates from someone using a computer who dials the number themselves from a phone. When the call is from a mobile device, or someone on a PC calls from the computer itself by clicking, the standard click charges apply. Previously, the calls dialed on a phone were free, as they didn't involve a click, per se.
"Call metrics was launched in Nov '10, and allows advertisers to see reports on phone calls coming through the unique phone number generated for ads with phone extensions. At launch, the company said the service was free but strongly hinted that this was likely to change."
"Starting in early May, 2011, we'll be making changes to pricing for calls completed to the Google-provided phone number displayed to potential customers using desktop and laptop computers (represented by the shaded row in the table below)," stated Google. However, Google didn't specify a date.
Only advertisers using call metrics have their phone numbers appear on PCs, otherwise click-to-call ads exclusively appear on mobile devices.
[Source: Help Center]