1. A method of monitoring the activity of a periodically sampled voice-communication line with conversion of each analog line-voltage sample into a binary word including a polarity bit, comprising the steps of comparing successive polarity bits from the sampled line with one another, counting the number of changeovers in the binary value of said successive polarity bits within a predetermined test interval substantially longer than a sampling period, deriving from the rate of change of said number of changeovers, counted in successive test intervals, a numerical value indicative of the character of a signal on said line, and comparing said numerical value with a predetermined threshold to ascertain whether said signal has the characteristic of a constituent of a spoken syllable.