The devices are narrowband notch filters so the effectiveness depends on which talkback frequency you are allocated to.
Dont equate having a non functioning talkback with not emmiting a traceable signal. The kind of test equipment that vm have access to will easily detect a signal 1/100th of the power required for a functioning talkback.
tbh, I dont know why anyone would choose to spend money on one of these devices when a quick cut with a craft knife will, in all cases, be far more effective !