This reports the voice codec currently negotiated for this line's active call -- for example G.711 or G.729 -- a field a tech checks when diagnosing choppy call quality, since a bandwidth-starved connection often forces a fallback to a lower-quality, more compressed codec. For example, seeing a call fall back to a more heavily-compressed codec than usual on a line with otherwise normal-sounding calls suggests upstream bandwidth congestion at the time of that particular call.