在呼叫程序上,本章主要介绍预测呼叫性质阶段。先是找到一个按钮收码器并把它与主叫用户接上。向主叫用户送出拨号音,接收按钮话机送出的双音频多频号码。该组合频率经数字滤波后,它代表的十进制号码被检测出来。收完用户送出的前三位号码后,对这三位号码进行预分析,由预分析确定这一呼叫的性质是本局呼叫还是出局呼叫之后,呼叫控制的预测呼叫性质阶段即结束。 In the calling procedure, this chapter mainly introduces the stage of predicting call nature. First find a button receiver and connect it to the calling user. Send a dial tone to the calling party and receive the dual tone multiband number sent by the push button telephone. After the combined frequency is digitally filtered, it represents the decimal number is detected. After receiving the first three digits sent by the user, the three numbers are pre-analyzed, and after the pre-analysis determines whether the nature of the call is the local call or the outgoing call, the call control predictive call phase ends.