Telephone subscribers can hear themselves through interlocutor. It sounds like echo effect on subscriber radio
1. Stereo Mix
Switch off Stereo Mix in the sound card settings.
2. Sound cards (for control station configuration only)
Dispatcher and radioserver installed on one PC should be using different sound cards. (You should have one additional sound card for each control station)
3. Microphone location
RX station / speakers are too close to the dispatcher's microphone.