The CPC-1 will generate a CPC disconnect signal on a PABX analog station (or any other line that does not provide this signal).
A CPC (Calling Party Control) signal is a momentary break in phone line loop current, which is used to signal automated telco equipment (ie. voice mail, call sequencers, etc.) that the distant party has hung up. Central Office telephone lines typically provide this signal, but analog PABX stations do not.
The CPC-1 will generate a 1 second disconnect when it detects a busy signal, reorder tones (fast busy), return to dial tone, or silence.