I have had this issue a few times now where you are unable to delete an extension because the system mistakenly believes a key is in use
Running command “state ext number” shows the state as busy
Busy reset completes but makes no difference
The trick is to “man busy” the extension with “busy ext number” then use “busy reset number”
The “state ext number” command then shows the extention as “Man busy” and you can delete the extension
Hope this helps
Found this answer here