Call tracking is the identification of an advertisement, a resource on the Internet or other advertising medium where the user saw information about the company and made a call.

How to track sources of incoming calls
There are two ways to trace the source of an incoming call: static and dynamic.

Static tracking is the assignment of one phone number to one advertising source or ad. When a customer calls the company, it is known exactly where he saw the ad.

The dynamic type of call tracking is the assignment of a unique phone number to each user during one session on the site. After making a call, the user becomes aware of:

from which ad on the Internet the user went to the site;
what key request found the company's ad;
what pages of the site viewed during the session;
how many times the user visited the site before making the call, or is this the first visit.

You can also ask customers in the old fashioned way how they learned about the company or give them to fill out a questionnaire. The method is good for maintaining a conversation, but not for serious analytics, users often don’t even remember where they saw the ad. The answer “on the Internet” is unlikely to help with the analysis of ten advertising campaigns.

The system for tracking the source of a call is called call tracking. With the help of a script installed on the site, information is collected about the user's transitions from advertising sources and his behavioral characteristics. The data is transferred to Google Analytics and stored in the personal account of the manager (an employee who analyzes incoming calls).

You can also find a large number of other tracking services such as I think this will help you increase your knowledge in this area.