This amplifier is designed for people with moderate to profound hearing loss or need to concentrate on their customers' needs. The HA-40 comes with a special button allows for even more amplification and eliminates background noise and feedback by muting the microphone in the handset. The Adjustable Tone Control lets you adjust the frequency of the incoming voice so that even similar-sounding words, such as “chair” and “share”, are crystal clear and easy to understand. The battery indicator light comes on every time you lift the handset to indicate that the battery level is high enough (battery included). This unit is also portable and connects easily between the handset and the phone base and includes slide controls for both volume and tone. Take it with you when you travel. It's also a great unit to use at work on analog or digital telephones.