The Excellent Quality Clarity HA40 Handset Amplifier - Plantronics Clarity Ameriphone HA40 Portable Telephone Handset Amplifier [01933-000] makes telephone conversations not only louder, but also clearer and easier to understand. Adjustable incoming volume control, Boost button allows for even more amplification and eliminates background noise and feedback by muting the microphone in the handset and Connects easily between handset and telephone base