If you're talking about Unity's call holding feature, it works like this:
Unity does a supervised transfer to the extension. To supervise, the original caller is placed on switch hold while Unity attempts to ring the extension. If it's busy, the caller remains on hold and Unity continues to attempt to ring the extension at regular intervals. During this process, the Unity port will definitely be 'in use'. Once the extension is finally answered, Unity will release the call and the port will be freed.
Any time Unity uses a supervised transfer (required for call screening or call holding features), the port will be tied up with the call until it's answered. When Unity uses release transfer, the port is freed up immediately after the transfer attempt. You'd use this transfer method if the phone system had some kind of hold queue for the Operator... but I don't believe that CCM has this functionality yet.
