Normal single-line telephones and key systems typically work on loop-start lines.
In an idle circuit, the central office supplies –48V (nominally) on the ring conductor with respect to the tip side.
A ground-start PBX initiates an outgoing trunk seizure on an idle circuit by connecting of the ring lead to ground (maximum local resistance of 550 ohms).
To avoid glare, before the PBX originates an outgoing call, it must first verify that the CO has not already applied ground to tip.
When the other end detects the loss of loop current, it also goes on-hook and the call clears normally.