The TCB-2 auto-answer coupler is perfect for listen-line applications. Just plug in the standard connectors, attach the power supply and you're ready to go. The coupler answers on the first ring and disconnects on hang up. 
 
Main Features/Specifications:
•Connections: Telephone line: RJ-11C 6-pin Modular Jack, Telephone set: RJ-11C 6-pin Modular Jack , Audio In/Out: 3-pin Male XLR
•Input/Output Impedance: 600 ohms
•Insertion Loss: 1 db
•Power & Physical: 15-24 volt AC or DC @ 50mA supply
•Size: 8.5" W x 5" D x 1.75 H (21.25 cm x 12 cm x 4.4 cm) •Weight: 1 lb 2 oz (.51 kg)
•Ringer Equivalence: 1.2B •FCC Registration: DXD6SN-14742-WP-N