• It tracks calls via the assigned Google Click Identifier (GCLID) set up in the advertiser’s account.
• It tracks calls via the conversion name for the advertiser’s store.
• It tracks calls via the phone number listed on the advertiser’s official store website.
• It tracks calls via a dynamically created Google forwarding number.

 The Correct Answer is: It tracks calls via a dynamically created Google forwarding number.

Explanation: Google’s phone call conversion tracking feature enables advertisers to ascertain the effectiveness of their ads in driving phone calls from their website. This is achieved by using a dynamically-created Google forwarding number that substitutes the business number on the site. The tracking counts a call as a conversion if it exceeds a set minimum duration, thereby omitting short, less substantial calls. Consequently, advertisers can pinpoint which specific keywords, ads, ad groups, and campaigns are inducing call conversions.

Read more here: https://support.google.com/google-ads/answer/6095883