Abstract: A facsimile machine (1) comprises an off-hook device (14) for ascertaining whether a telephone set (2) connected to a telephone line (TL) in parallel with the facsimile machine (1) has already taken an incoming call, which call is featured by a ringing signal in the telephone signal (TS). For this purpose, the off-hook device (14) compares the voltage value of the DC component (G(TS)) of the telephone signal (TS) with off-hook thresholds (OSW1, OSW2) and, if necessary, produces off-hook detection information (ODI1, ODI2) or fax off-hook information (FAI). Since the voltage values of the DC component (G(TS)) of the telephone signal (TS)—more particularly in a private branch exchange—may be lower than specified, in response to which the off-hook device (14) could determine a false outcome, the off-hook device (14) activates an error mode when the off-hook detection information (ODI1) is produced simultaneously with the ringing signal detection.