This handset amplifier works fairly well, but has two serious flaws that can sometimes make it difficult to use. In order to extend battery life, the amplifier turns off after 10 seconds without a strong incoming signal to amplify. As others have commented, this is a nuisance when you are on hold (music on hold usually isn't enough sound to keep the amplifier on) or there are frequent pauses on the far side of the converation.The other flaw is that the specified batteries seem to have a limited life even if the amplifier isn't used often. The manufacturer specifies three AC675 zinc-air (air-activated) hearing-aid batteries. Perhaps these cells work best in a hearing aid, where they are used every day for hours at a time. I've found I have to replace the batteries in this amplifier about every six weeks.