Hillian1
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Very. I work as a broadcast engineer at the moment, having a sound basis in audio processing (and I used to work for BT, although that probably reduces the value of anything!).
Cheap IP voice circuits, as being used for this kind of work, carry voice frequencies and voice frequencies alone; usually a vocoder codec that simply does not carry anything that's not likely to be a voice. They aren't going to hear a whistle as you hear it; nor will they hear it at the volume you're hearing it. You're likely damaging yourself more than there was any chance of doing anything to them.
Oh.
How very annoying.