This issue occurs in Cisco Unity 4.x when subscribers do not have a recorded name.
In order to resolve the issue, complete these steps:
Go to the Unity Admin and search for the missing subscriber.
Go to the subscriber's profile page and verify that the List in phone directory checkbox is checked.
Also in the profile page, verify that this subscriber has a recorded voice.
If there is not a recorded name, the recording tool bar will show 0.0 -- 0.0. If there is not a recorded name, the easiest option is to check Set subscriber for self-enrollment at next login and then record the name.