Sometimes a user calling your website may not dial your call tracking number straight away. They may, for example, take note of your number and call it later. If users do not call in the time allotted for their session (approximately 5 minutes), then their tracking data (keywords, phone number, referral, etc.) will not be associated with their call.
In this case, you would not see the tracking data appear in your reports.
Please visit this page to see some solutions on fixing this issue.