This unit will visually alert when phone rings by flashing a bright red light. It will also audibly alert by ringing an internal electronic bell. It works from the phone line voltage. (Will NOT work with wireless or mobile phones.) No batteries or external power source is required. Excellent for recording studio, baby nursery, the hearing impaired, noisy factory or shop environment or when the TV/Stereo is at high volume. It is easily installed on modular phones. Simply plug into wall jack and plug the phone into the unit. It has switch on the side for flasher only or flasher plus ringer. Unit is in excellent condition, and includes both cords. One cord goes into the wall jack and the other goes into your phone; however, you don’t need to plug your phone into this if you don’t want to. It can be used as just a standalone flasher/ringer. I will sell for $9.