Methods, systems, and apparatus include computer programs encoded on a computer-readable storage medium for dynamic telephone number assignment. A method includes: providing a content item to a user device, the content item including a control for initiating a call to a content sponsor associated with the content item; assigning a telephone number from a pool to the content item; receiving a control interaction indication; facilitating a connection between the user and the content sponsor based on the interaction; storing an association between a user identifier, the telephone number, and a content sponsor identifier such that future calls from the user to the telephone number can be routed to the content sponsor, even after the telephone number has been returned to the pool; and linking a content item impression with the connection such that a report can be generated that shows an effectiveness of the content item impression.