Methods, systems, and apparatus, including computer programs encoded on computer storage media, for number identification and tagging. One of the methods includes identifying a content item to be provided to a mobile device. The method includes identifying a location of the mobile device. The method includes identifying at least one calling format based on the location. The method includes identify a calling number included in the content item based on the at least one calling format. The method includes generating an altered version of the content item to tag the calling number. The method also includes provide the altered version to the mobile device.