The Clarity HA40 Portable Telephone Handset In-Line Amp is a great, easy solution to modify phones that have a handset separate from the keypad so they are more hearing-friendly. It easily installs between the handset and the base and will increase incoming sounds up to 40 dB! It also has an adjustable tone control to help distinguish between similar-sounding words like "chair" and "share" as well as a boost button that mutes the microphone in the handset to eliminate background noise and feedback. Because of its small size, this easy to use telephone amplifier is ideal for traveling, as well as at the home or office. So make your incoming telephone conversations louder, clearer and easier to understand with the HA40 amp!