Systems and methods of estimating call conversion rates are described herein. The system can include a conversion tracker, which can receive, from a client device, an identification of a conversion event at a first time. The conversion event can be associated with a content item, content item provider, and phone number of a content item provider. The conversion tracker can transmit a request that can include the identification of the conversion event. Once received, the client device can scan the call log for the phone number and send an response indicating the phone number's existence. The conversion tracker can determine a time window in which the client device called the content provider. The conversion tracker can also aggregate the responses across multiple client devices to estimate the call conversion rate for the content item associated with the content provider.