How Google adwords website call conversions work?

There are two common types of call tracking options:

1. Session-Based, where every visitor coming to the site receives a unique phone number based on the referring URL.
2. Rule-Based, where you only use a unique phone numbers for specific campaigns.

Thank you for inviting me to answer the question.  Mohammad nayeem uddin answered it correctly.  Post here if any further clarification you require. 

First of all this feature is available only to advertisers in the US,Canada, UK, France, Germany and Spain.

Google provides a snippet of code you can add to your desktop or mobile site. A unique Google forwarding number is generated for each AdWords ad click and will continue to display to the user for up to90 days so you can track conversions on any future calls from that visitor. You can customize the color, font and size of the numbers as they appear on your site, to ensure they’re consistent with your overall design.

Once the code is working, results are available in conversion reporting and in Call Details. Google will display the number of calls you’ve received and other details including the call length.

This image explain the flow.

Let me know if you need more info. Thank you.