In order to ensure that you have accurate reporting, you may want to deny any callers that are calling from a private number. That is, a number that is hiding their caller ID. When callers ring in on a private number, it is impossible for you to name them in the portal. To combat this issue, you may like to deny all unknown callers. This will force them to dial the "Unblock prefix" which will make their phone number visible to you. For more information on this can be found here

Log into the Vattendance Portal

Next to the PBX Status heading, click "Edit"

Find the "Deny Private Callers?" box and change it to "Yes". Then click "Update"