Door Unlock Contact Closure (Strikeplate Required), Transmit and Receive Volume Controls, Alarm Functions, Background Music Input With Volume Control, Built-In Ring Generator With On/Off Switch, 3 Distinctive Ring Patterns; Programmable Options Include: Door Unlock Codes, # Of Ring Cycles Before Door, Request Is Terminated; Time The Door Remains; Unlocked (Per Door), Places Simultaneous Calls In Queue (V-2904),Requires -24 Vdc, 600mA (Filtered) 10 Vac Lamp; Access: Electronic or 1A2 Line Key (line card required, PBX Loop Start Trunk Port, or Dedicated Single Line Phone SetSpecifications: Input Impedance: 600 ohm; Output: 45 ohm; Music Level: -10 dBm nominal: music Input: 4 to 600 ohm; Power Requirements: -24 Vdc, 600 mA; Shipping Weight: 4.8 lbs (2.18 kg); Dimensions: 8.6"H x 11.0"W x 2.3"D (21.84cm x 27.94 cm x 5.84 cm)