The echoes are usually caused by the looping of recorded sound. This happens quite often when the devices are situated too close to each other. It is possible that your Dog Station is picking up the sound coming from the Person Station.
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Please make sure that you place the Person Station out of the Dog Station's hearing distance. After that, this issue should disappear.
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You can lower the microphone sensitivity within the app (Menu -> Settings -> Microphone sensitivity) and the output volume of your phone.