A special boost button maximizes amplification while blocking out feedback and background noise. Unit is compatible with most home and office telephones and is easily connected between handset and phone base. Uses 9V battery (included). A great secondary or backup amplifier at home, work or while traveling. One year warranty. Clarity Power technology. Tone control Boost button maximizes amplification while reducing background noise. Compatible with most corded home and office phones. Will not work with Trimline telephones or phones with a dial pad in the handset. Uses 9V battery (included).