A tone detection method for a telephone subset to detect tones in the presence of noise and speech. According to the method the period of an input signal containing the valid tone is measured at a fixed sample rate (say 10 ms). The measured periods are compared to that expected of a valid tone and a pass/fail decision is made for each sample and stored in a buffer whose length is chosen to be longer than a complete cadence cycle of the valid tone (say 1 second). By examining the buffer it is possible to calculate a percentage pass rate for any period. If the pass rate is above a predetermined threshold then a valid tone is present.