Having launched her first Search campaign, Asha wants to know just how many calls are being generated from the number in her ads, on her website, or clicks on a phone number on her mobile website. What conversion source should she track?
The correct answer is:
If phone calls are essential to your business, you can use conversion tracking to help you see how effectively your ad clicks lead to different kinds of phone calls.
Read more here: https://support.google.com/adwords/answer/6100664