This can sometimes happen when the line gets 'hung' on your system.
When you try to use the line it shows, in use, by another extension or additional application such as voicemail although you can still receive incoming calls on it.
The hung line can usually be cleared in your systems programming depending on which Norstar system & software level you have. As this requires access to your systems maintenance programming, we suggest you contact the Norstar Adviceline who will be happy to talk you through the programming to clear the hung line.
Note: Applies to Companion 10