"Pass Limit" error message on Cisco VoIP Phone

I recently plugged in some Cisco 7962 VoIP phones to upgrade their firmware and prepare them for deployment. After the firmware upgrade, a few of the phones were missing the auto-registered directory number (DN) and there was a "Pass Limit" message at the bottom left corner of the screen.

Solution #2 on this page showed me that the problem was related to unassigned DNs. To resolve the issue, I:

1. Unplugged the phone
2. In the Cisco Unified CM Administration site, went to "Device" -> "Phone".
3. "Find Phone where Device Name begins with SEP". I replaced with the hardware address of the phone that wasn't auto-registering.
4. Clicked the "Find" button.
5. Selected the box to the left of the phone.
6. Clicked "Delete Selected".
7. Plugged the phone back in and it auto-registered normally.

To prevent this from happening, I should clear out my unassigned DNs periodically:

1. In the Cisco Unified CM Administration site, go to "Call Routing" -> "Route Plan Report".
2. "Find Unassigned DN Route Plan Report where Pattern/Directory Number begins with ". just means leave that field blank.
3. Click the "Find" button.
4. Click "Delete All Found Items" in the "Route Plan Report" menu bar.

Models and versions: Cisco CP-7962G VoIP phone with firmware SCCP42.8-5-3S, Cisco Unified Communications Manager (CallManager) 6.1.5.10000-10

WARNING: This worked for me, but there is no guarantee that it will work for you. Please understand what you're doing before you change anything.