由内线接电话终端，数字线进行局间互联，从而构成电话网。收到呼入，呼出请求后，通过查询路由表来决定是内部通话还是和外部通讯，然后通过通道交换功能实现通道间相互通话-from inside to answer the phone terminals, digital lines between the Internet Bureau, thereby constituting a telephone network. Call received, exhaled request, the routi