Using the one-button frequency scanning function of the Sennheiser GT12, I detected a simulated sub-audio signal from the Vale C50 at 5673.5 Hz. This implies that the maximum simulated sub-audio frequency should be 250.3 Hz. However, this results in the GT12 being able to hear the C50 speaking, but the C50 cannot hear the GT12 speaking. What is going on here?