Camp on Busy
From Zenitel Wiki
If a station or a feature that you call is busy, you will hear a busy tone and your display (if any) will show "busy".
The ICX-AlphaCom holds a feature called Camp on Busy. If the station or feature becomes free within a pre-set time limit (default 30 seconds), you are automatically connected.
If not, the call is ended and the Call Statistics Log will show Call Busy End.
Getting a Busy signal can be avoided by queueing calls using the Ringing Group Feature.