This specification relates to tracking phone calls related to online content items. In general, one aspect of the subject matter described in this specification can be embodied in methods that include the actions of determining a probability that a call to a phone number resulted from a first interaction of a set of interactions. In general, another aspect of the subject matter described in this specification can be embodied in methods that include the actions of determining a rank score for an online content item, including for each call of a plurality of phone calls to the phone number, attributing the call to an online content interaction of a set of online content interactions; determining, using the rank score, whether to provide the online content item to a client device; and providing the online content item to the client device upon determining to do so using the rank score.