Look on page 18 of the attached PDF: Matrix Plus/AUX (Rear Panel, DB-15M)The Matrix Plus/AUX connector is intended for directly connecting a Clear-ComMatrix Plus system. Other 4-wire types of intercoms can also be used via thisconnector. The following is a description of each group of pins on theconnector.
+/- TALK (Pins 2 & 10): The TALK pair is a 600 ohm (0 dB level) transformerbalanced output that is active any time the TALKBACK button is active.
+/- LISTEN (Pins 3 & 11): The LISTEN pair is a 10 Kohm (0 dB level) transformerbalanced input that is selected with jumper and dip switches. Refer tothe application described on page 17.
CALL OUT (Pins 5 & 13): This pair of pins is an opto-isolated output intendedto activate the CALL signal input of a Matrix-Plus port input wheneverthe TALKBACK button is pressed.
CHASSIS (Pin &): Chassis ground of the AB-100.24 VAC POWER INPUT (Pins 8 & 15): A second method of entering externalpower into the AB-100. These pins are parallel to the 2.1 mm coaxial powerconnector that is normally used with the wall mount transformer.
1~ NO CONNECT9~  NO CONNECT
2~ +TALK10~ -TALK
3~ +LISTEN11~  -LISTEN
4~  NO CONNECT12~  NO CONNECT
5~  CALL OUT COMMON13~ ( CALL OUT
6~ NO CONNECT14~ No Connect
7~  CHASSIS GROUND15~  24 VAC POWER INPUT8~  24 VAC POWER INPUT

Viewed from the rear of the connector