Device and method for determining tone ringing frequency

The invention relates to a method for determining tone ringing frequency which comprises the following steps: Forming a zero crossing signal from a tone ringing signal by comparing the tone ringing signal with a threshold (S), whereby the zero crossing signal has a succession of alternatively rising and falling edges between two zero crossing signal values; measuring the respective duration between the adjacent rising and falling edges of the zero crossing signal; comparing the measured duration with a predetermined duration limiting value (tg); defining an evaluation start time (t1) if a measured duration is greater than or equal to the duration limiting value (tg), whereby the evaluation start time (t1) is the instant of the subsequent edge; defining an evaluation stop time (t2) if a measured duration with an identical zero crossing signal value for the time occurring after the subsequent time is greater than or equal to the duration limiting value (tg), whereby the evaluation stop time (t2) is the instant of the subsequent edge, and; determining the frequency (f) using the measured time difference between the evaluation start time (t1) and the evaluation stop time (t2).