With a business in an eligible country, advertisers can use Website Call Conversions to track calls to a Google forwarding number on their site from users who arrived there from any source.
Call conversions, which can be tracked by businesses in eligible countries, can be used to track calls from users who visited a website from any source
Google provides a unique phone number that’s displayed with your ads. If a potential customer calls this phone number (Not just arrive on website from any source), AdWords will route the call to your business phone number. You’ll then be able to see detailed reports about calls generated from your ads.
Read more here: https://support.google.com/adwords/answer/2382961?hl=en