Suppress Ringtone

Hi, when we signal an incoming call with callCommand.IncomingCall(call, null, CallRingTone.RingTone_Unknown, CallAudioRoute.AudioRoute_ToHeadset) in C# using the APU-75, Plantronics plays a ringtone (a beeping sound) and the audio stream is silent, but we would like to play our own ringtone instead. To that end, we must disable the ringtone of Plantronics somehow. Calling device.HostCommand.SetRing = false before or after the above line does not help because the ringtone is played anyway (at least for some seconds). Is there a way to disable the ringtone for an incoming call?