The following tips can help the user troubleshoot nulling issues when adjusting the R (resistance), L (inductance) and C (capacitance) controls on the front panel of the CCI-22 interface card:
First, assure that no one is using the system. This is for two reasons. One is that there will be a series of beep tones that might seem annoying to anyone trying to use the system for communications. The other is that, to get a good null, we need to assure that all talk paths are not engaged in the system relating to this interface. To null a CCI-22 interface connect a standard 3.5mm earpiece (or MP3 headset) to the Null socket on the front of the unit. The user will hear a square wave. Adjust the R,L,C pots on the front of the CCI-22 for minimum tone. ensure that all talk paths on the PL system and matrix are turned off.
1. If the "R" control is turned fully counter-clockwise, the line has either more than one termination, or an excessive resistive load.
2. If the "R" control is fully clockwise, then the line has no termination.
3. The "L" control compensates for the low-frequency inductive and capacitive elements of the external party-line. If the "L" control is fully turned in either direction, it is likely that there is a problem in the external party-line. When a Clear-Com party-line is connected, the "L" control should be just to one side of its mid-pot position.
4. The "C" control compensates for cable capacitance; the setting depends on the length of the line. If the "C" control is fully
counter-clockwise, this indicates a very short line (under ten feet); this is a valid setting for a short line.
5. If the "C" control is fully clockwise, this indicates an excessively long line (over 4,000 feet or 1200m).
The picture below shows one channel of the CCI-22 nulling controls.  A small set of earbuds can be used to activate the manual nulling square wave signal and minimize the test signal when adjusting the R, C and L controls. 
You should adjust the R, L, C pots in the following order R>L>C, and then repeated again, two or three times, until the deepest null can be achived. So you are looking to get the tone as quiet as possible. 






Then repeat the process on the other channel, if it is being used.