The IntelliCall Telephone Amplifier is an amplifier designed for use by individuals who are hard of hearing. This device connects to standard corded phones and amplifies incoming voices 45 decibels or more on a handset or headset and can be used with any headset with a RJ9 plug (the same as telephone handset-cord connector). High Definition Sound (HDS) Technology eliminates virtually all distortion, echo effects, and unwanted static and digital tone enhancement boosts and enriches the speech frequencies that are difficult to hear. The automatic volume control optimizes sound level. POWER: Uses the included alternating current (AC) converter and a nine-volt battery (not included). GUARANTEE: 90-day satisfaction guarantee.