Select Yes to prevent phones from receiving calls when they are already engaged in a call. The incoming calls are immediately forwarded to an alternate destination such as voice mail. If an alternate destination is not available, the system returns busy tone.
This option applies only to line keys that the device "owns". Keys a device owns are:
appearances of its own prime number
a phantom DN programmed no where else in the system.
Phantom DNs appearing on multiple devices are thus exempt from the "Force Device Busy If Any Line In Use" option; calls to the DN will alert all devices with an appearance of the DN whether or not the device is busy.
Furthermore, enabling this option effectively turns a multicall group into a one-busy-all-busy group.
NOTE: This option does not apply to Mobile Lines on 6900 series phones.
Think it will work fir 5300 series, seems new phones not...WTF