八十年代初，我国公用电话系统的供求比较紧张，为了满足单位内部各部门的通讯需要，许多单位纷纷建立自己的内部交换网络，通过少量的中继线路或者用户线路与外部单位联络。随着公用电话系统的不断完善和通讯技术的提高，我国的公用电话已能满足用户的需求，为了减少已经投入? In the early 1980s, the supply and demand of public telephone systems in our country was relatively tight. In order to meet the communication needs of various departments within the unit, many units set up their own internal exchange networks to contact external parties through a small number of trunk lines or subscriber lines. With the constant improvement of the public telephone system and the improvement of the communication technology, the public telephone in our country has been able to meet the needs of users. In order to reduce the input already invested?