I have the setup that Mushfiq mentions for a couple of lower paying accounts. The rest have a callrail integration. This puts a minimum of 4 numbers on the website, called a pool, that can show on your site depending on the location and user. Connecting it to ads allows it to create its own conversion action for you and measure those calls, including those that clicked an ad but didn't call until much later. You choose the length of time for it to "remember" the user's cookie in the callrail setup.