Plim81,
I am assuming that you are referring to when outside callers call you directly on your DID/DDI? That would be the DID/E&M Receive Busy Instead of Camp-On flag under your phone.
System > Devices and Feature Codes > Phones > {your ext} > Flags > DID/E&M Receive Busy Instead of Camp-On: Yes
The only other Busy condition state would be when you put your phone in DND and that flag would be Receive Busy Instead of DND.
System > Devices and Feature Codes > Phones > {your ext} > Flags > Receive Busy Instead of DND: Yes
If you are referring to something else then you will need to be more specific on what Call Type and Condition you are wanting this to occur on.
Thanks,
TE