My phone displays the incorrect extension.

This will happen if you swap phones with someone. This is not recommended. You'll have to call the Telephone Help Desk, 74663. They will need to re-record your MAC address on the phone (centre sticker on back of phone near network cables). Before you call the Telephone Help Desk, you will need to locate whoever has your extension (the person you swapped phones with) so that CCS can change both phones appropriately.