Deleting an Accidental Duplicate Extension

I inherited a Free-PBX system when I started a new job. I’ve nearly got my head around the system except I’m stuck with a mistake I made. We are using Aastra phones with the Aastra.XML scripts. The other day a user who was assisting me mistyped a new extension number in and it logged in using another user that already has a phone. That particular users phones is still up and running but when start up the new phone that had his extension typed in also, it logs in as his extension. I’ve tried logging into the phone and deleting the local config and resetting to factory defaults. But to know prevail. Is there something in Free-PBX I can do, so it resets the phone allowing me to type in the correct extension again, from what I can see Free-PBX is remembering the particular phone using its MAC address and giving it the incorrect extension!?
All help would be most appreciated.

Yep, you need to release the phone:

Aastra Scripts Admin Manual (not support by FreePBX)