Google is now using the technology behind Google Voice to measure the phone calls that Adwords generates for an advertiser.
This new feature is called Call Metrics and works by assigning a unique phone number for each campaign and inserting it as an additional line of ad copy for desktop and “high-end” mobile devices. The calls are routed to your business phone number, which you select and are then tracked in the interface.
This new feature is only available to a limited number of advertisers, however you can check out your Ad Extensions Tab to see if this option has been made available to you.
Call Metrics will allow you to track the amount of calls (and duration) to your business as result of your Adwords efforts. Even though Google plans to charge for Call Metrics in the future, being able to more accurately calculate your return on investment will allow for a more refined strategy and allocation of budget.