Type of tone: off(1) no dial tone detection single(2) detect just one tone rhythm dual(3) detect also an alternative tone rhythm Enable dial tone detection. The following entries describes the parameters of the expected tone. It consists of time, level, frequency, rhythm and tolerance values. There are legacy central offices in some countries which uses differently sequenced tones. So beside the original tone description an alternative description can be used to detect those tones also. Another aspect is the allowed deviation from the expected tone rhythm which is quite different in the national telephone networks. To take this fact into account the tolerance entries determines how precise the given rhythms should be proven.