Who is calling? 谁打电话来？
Whom do you want to speak to? 你想找谁接电话？
作主语时用who，作宾语时用whom，在口语中用who作宾语时也不少：
Who did you mean? 你指的是谁？
Who are you talking about? 你们在讲谁？
直接跟在介词后时只能用whom：
With whom did you come? 你和谁一道来的？
(口语中说“Who did you come with?时更多一些。)

2. 其他三个疑问代词可用作：
1)主语：
What’s your address? 你的地址怎样写？
Whose is better? 谁的好一些？
Which of these is yours? 哪一本是你的？
2)宾语：
What do you mean? 你是什么意思？
Which do you like better? 哪一个你更喜欢一些？
Whose have you chosen? 你选了谁的？
3)表语：
What’s her name? 她叫什么名字？
Whose is it? 这是谁的？
4)定语(这样用时有些字典标作形容词)：
What time does the train leave? 这列火车什么时候离开？
Which room shall we stay in? 我们住哪间房？
Whose passport is this? 这是谁的护照？