Camp On tones on the receiving phone can be disabled by enabling COS Option "Camp-on Tone Security/Fax Machine"
Select "Yes" to prevent camp-on tone from being presented to a station. Although the camp-on feature is not disabled by selecting this option, any audible notification is disallowed and camp-on is applied without intrusion on the called party's line.
Selecting "Yes" also removes compression (G.729a) as a codec choice. Devices with this option enabled will default to G.711 as the codec.
There is an island of opportunity in the middle of every difficulty.
Miss that though, and you're pretty much doomed!