Call conversions, which can be tracked by businesses in eligible countries, can help advertisers understand the value that calls from their ads are driving by counting calls of a minimum duration to a Google forwarding number as conversions.
This can be understood with a following example. Let’s suppose a travel agent notices that she’s gotten more calls since adding call extensions, and wants to better understand this phone traffic. She turns on call reporting. Reviewing the data, she sees calls lasting more than two minutes tend to lead to sales, so she chooses to count those calls as conversions. Later, she sees that her conversions are coming from callers in the 717 area code, so she decides to target her ads to the corresponding region.
Read more here: https://support.google.com/adwords/answer/2454052