When users call this number (either from viewing on a smartphone or from their desktop computer), Google connects the user to the advertiser and tracks the length of call and where the call originated from.
If Google roll out this feature, it will mean that advertisers will not need to source a number of unique telephone numbers for tracking themselves and Adwords will include more detailed call metrics.
More info here.
This is currently only available in US and Canada but I can already see the reporting feature in my UK accounts:
There are 4 ‘Call details’ columsn available on both the Campaigns tab and Dimensions tab in the Adwords U (additionally on the Dimensions tab you can view call start time, end time, caller area code and missed or answered status). These call metrics have been given names that fit well with existing PPC metrics, therefore Phone Calls is the total number of calls received to the Google phone number – equivalent to clicks. Phone impressions is the number of times the phone number was displayed on devices, so the same as impressions (apparently this figure though is only for devices where the phone number was displayed but cannot be directly called from the device). Phone through rate (PTR, like CTR) is the number of phone calls received divided by impressions. Phone cost is the total cost of calls manually dialed to the Google number but this does not include charges from clicks on a phone number displayed in ads shown on high-end mobile devices.