The problem with these intercoms comes from the setting a station number and selecting which station you want to talk to

I tried swapping units around, changing channels, etc, but could never isolate the problem to a single defective unit; it seemed to happen to all of them.

works great no problems

We use it in an office and the lower level is all concrete walls which we've had problems using intercoms like this

But I had a significant problem with the product when I first received it