AUTOMATIC DIAL TONE DETECTING METHOD

PURPOSE:To surely detect a second dial tone in a method for detecting the second dial tone from a DDX network with a telephone line. CONSTITUTION:Dialling is executed to the DDX network after detecting the first 400Hz continual dial tone, a first timer TIM 1 is set by the making state of a 400Hz signal when the second 400Hz continual dial tone from the DDX network is detected, the second timer TIM 2 is set by the breaking state and the third timer TIM 3 is set by the first cycle and the third cycle making state so that it is judged that the second dial tone is detected(DTON '1') when the state becomes the breaking one after the time-over of the first timer TIM 1, the state becomes the making one after the time-over of the second timer TIM 2 at the same time, and also the state becomes the third or fifth cycle making state before the time-over of the third timer TIM 3.