Clear-Com HX System Frames - Manual (Page 28)

Pair Description
Pair 1 Pair 1 is not used.
Pair 2 Transmits digital data from the four-wire device back to the
matrix
Pair 3 Transmits audio from the four-wire device to the matrix.
Pair 4 Pair 4 is not used.
Table 6: 2-Pair analog
Figure 5: Connecting the matrix to a four-wire device (2-Pair analog)
28 Eclipse HX-PiCo User Guide