IMPORTANT: This 2-piece kit (ring sensor and extender arm) is included with the HL10 handset lifter so there is no need to purchase it separately if buying the lifter. If you have lost or misplaced these parts, this kit is sold as a replacement. Primarily used with Northern Telecom ("Nortel") Meridian or Norstar telephones. Ring sensor plugs easily into the accessory port on the HL10 lifter and adheres to the ring speaker of your telephone. Note: This kit also comes with extender arm for Nortel M3900-series, and other phone models which have a curved side design.