Instruct: Classify a given review as either positive or negative.
Query: Questionable product

I bought three of these units plus a doorbell. Sometimes they work, sometimes they do not, and they create a lot of static noise. Sometimes a unit emits noise even when it is not in use, which can be quite annoying. I investigated these problems and learned that the units need to be connected to outlets wired to a single phase to work. If they are plugged into outlets connected to two different phases they may not work. This explains why the units work in some locations but not in other, but does not explain why a unit connected to one outlet sometimes work and sometimes does not. Furthermore, the system is prone to interference from electrical appliances. Since most homes have three phase wiring and use a lot of appliances that can generate interference, this suggest a faulty design concept.