Ring Generator, Built-In Talkback Amplifier, Transmit and Receive Volume Control, Provides Two-Way Communication (Does Not Provide Door Unlock Capability), Compatible with all Valcom Doorplate Speakers, Power Supply Included (VP-324); Access: Electronic or 1A2 Line Key (line card required, PBX Loop Start Trunk Port, or Dedicated Single Line Phone Set; Specifications: Input Impedance: 600 ohm; Output: 45 ohm; Power Requirements: Power Supply Included; Shipping Weight: 3.4 lbs (1.54 kg); Dimensions: 8.25" H x 4.5" W x 2.35" D(20.95 cm x 11.43 cm x 5.96 cm)