From the 3300 Help files:
Camp-on Tone Security
Description
Camp-on Tone Security is a Class of Service option that suppresses the application of Camp-on tone. If any party in a call has this option enabled, no Camp-on tone is returned to anyone in the call.
Ports connected to fax machines and trunks (except IP trunks) used for fax calls should have this option and the Override Security option enabled in their COS.
Conditions and Feature Interactions
If you attempt to Camp-on to a busy station that has Camp-on Tone Security enabled, you will hear special busy tone instead of Camp-on tone or Music.
Programming
Enable Camp-on Tone Security and Busy Override Security in the Class of Service Options form.