High Volume 40 dB External Telephone Phone Ringer Amplifier Good presents for Graduation, Birthday, Christmas, Anniversary, Mother's Day, and Father's Day Great gift for friends, families, co-workers and bosses This HA40 Clarity brand in-line telephone amplifier is a battery-powered phone amplifier with slide controls for both volume and tone, providing up to 40 decibels of amplification for telephone users with hearing loss. The tone selector amplifies specific frequencies for optimum speech clarity and distinction between similar sounding words, allowing users with a hearing loss to customize the assistive listening device. The HA40 telephone amplifier has a special boost button to maximize amplification while blocking out feedback and background noise. The HA40 is compatible with most home and office telephones and is easily connected between handset and phone base. The HA40 uses a 9V battery (included), and is a great secondary or backup amplifier for amplifying a telephone at home, work or travel.