I think you are referring to CPP bidding also known as bid-per-call. I would recommend checking the following link for more information -
To get started in the most simple form you can go the Ad Extensions tab, click the View: drop down button and select Call Extensions. Click on +New extension and you will be presented with a field to enter your phone number along with different options, the link included above should help you select the appropriate options.
Let us know if you have other questions!