FCC registered unit will visually alert when phone rings. Works from phone line voltage. Will NOT work with wireless or mobile phones. No batteries or external power source required. Excellent for the hearing impaired, noisy factory or shop environment or when TV or Stereo are at high volume. Easily installed on modular phones. Simply plug into wall jack and plug phone into unit. Has switch on the side for flasher only or flasher and ringer. Ringer is not louder than normal telephone ringer but provides an additional ring.