Today ' s familiar telephony features - - such as caller id , call waiting and 800 / 888 numbers - - are intelligent value - added services made possible by a protocol called signaling system 7 ( ss7 ) 今天人们熟悉的电话功能,如主叫识别、呼叫等待、 800 / 888业务等,都是由于有了七号信令系统协议而实现的智能增值业务。