When a user visits a website in response to selecting an advertisement, information about the user and/or about the user's visit to the website can be obtained. During the user's visit to the website, a tracking phone number can be displayed. If the user calls the tracking phone number to complete a purchase, an identifier can be embedded in the phone call. A system that handles the phone call can extract the identifier and use it to uniquely associate the phone call with the information obtained about the user's online activity. The system can also use the identifier to obtain the information so it can be used to customize the routing and/or handling of the phone call. The information may also be employed to dynamically determine if or when to present a tracking phone number to the user.