这里明明是表示“电话占线”，怎么用“人”来作主语呢？

表示“电话占线”时，be engaged 的主语可以是“人”，也可以是“电话”，也可能是“电话号码”，也可能是“电话线路”。如以下各句均可表示“我们试图和老板联系，但他的电话占线”：

We tried to reach our boss, but he was engaged.

We tried to reach our boss, but the phone was engaged.

We tried to reach our boss, but the number was engaged.

We tried to reach our boss, but the line was engaged.

另外，表示“电话占线”，除用 be engaged 外，也可用 busy，但此时不宜用“人”作主语，否则容易发生误解：

I called Sonya, but her line was busy.  我给索尼娅打过电话，但是占线。

The line is busy—I’ll try again later. 占线 —— 我过会儿再打。