The CCI-22 is a 2-WIRE CLEAR-COM / RTS PARTY-LINE INTERFACE (DUAL). The CCI-22 connects two 2-wire party-line channels with the intercom matrix. The interface supports Clear-Com signaling to and from the matrix system
POWER LEDs
The green “PL Power” LEDs indicate whether the interface channels are getting +30 VDC power from the external party line to which they are connected.
INPUT / OUTPUT AUDIO LEVEL CONTROLS
The “Send” level controls affect the level of the audio signals from the matrix to the external party line.  The “Recv” level controls affect the level of the audio from the party line to the matrix. The level controls have a range of ±13 dB; they are normally set to the midrange position.
AUDIO REJECTION NULL ADJUSTMENT
Sidetone is the sound of the operator’s voice in his headset. In interfaces, it is necessary to null (minimize) the sidetone as much as possible when an external party line is placed in the matrix environment. Ideally, there should be no portion of the talk signal in the listen signal. 
Separate “R” (resistance), “L” (inductance), and “C” (capacitance) controls compensate for each component of the line’s impedance, providing the best null possible.  The null circuit is effective on line lengths between zero and 4000 ft (1200m) with impedances in the range of 120 to 350 ohms, and can reduce local audio in the received signal by more than 30 dB over the frequency range of 200 Hz to 8 kHz.
To null one channel of the CCI-22:
1. Connect the external party-line devices to the CCI-22 channel. Make sure that any connected devices do not have their microphones on.
2. Plug the accessory earphone into the front-panel jack labeled “Test.”  This will disconnect the interface from the matrix and enable a test oscillator. The oscillator produces a square wave with both low and high harmonics, allowing testing of all frequencies. The test tone pulses approximately every 0.5 seconds.
3. While listening to the test tone in the earphone, adjust the “R” control until the tone is at a minimum.
4. Repeat Step 3 with the “L” and “C” controls. Because these controls interact, steps 3 and 4 will have to be repeated several times until the test tone is minimized. Continue adjustment until the tone is minimal. If a deep null cannot be obtained, it is likely that something is wrong either with the wiring in the external party line or with one of the other devices attached to it or may indicate that no termination is set / too many terminations are set on the individual PL line. 

Visit the Clear-Com web site for more information on the CCI-22