变进制电话交换机

step-by-step step by step telephone switching system switching carrier system telephone a thousand-li journey begins with the first step--the highest eminence is to be gained step by step telephone switching system telephone channel wire telephone telephone instruction step-by step antomatic telephone system step-by-step system step by step system step-by-step automatic system step-by-step autometic system telephone switching system automatic telephone switching system step-by-step operation step by step exchange step-by-step procedure step by step welding