These Work Great
I bought two of these nearly 6 years ago to use with our radio stations remote broadcasts. Our sports announcers call the number, the box auto-answers the call and feeds station audio down the line back to them. We can do game remotes with no one at the station. They have never failed us, sound great and have even survived a station lightning strike or two. And yes, the null adjustment actually does what it's supposed to. Plus, they're remote controllable.