It is normally a repeated tone, designed to assure the calling party that the called party's line is ringing.

这种声音一般都是一个重复的音调，目的只是提示打电话的人，对方的电话正在接通中。

youdao

This paper introduced the principle of calling party number identifier (CID) and its design in the multi_functional telephone.

分析了主叫号码显示原理及其在多功能电话机中的设计。

youdao

This paper introduced the principle of calling party number identifier (CID) and its design in the multi_functional telephone.

分析了主叫号码显示原理及其在多功能电话机中的设计。

youdao