When my sister phoned me, I could not hear clealy what she was_____.

Speak 意思是“讲话”,强调说话的能力,方式和对象,即可以作及物动词也可以作不及物动词.作及物动词时,常用表示语言的名词或代词作宾语.例如：
Look!She is speaking in the classroom.瞧,她在教室里讲话呢.
Do you speak English?你讲英语吗?
Say 意思是“说”,强调说话内容.例如：
"What are you saying?""I'm saying nothing."
Talk 意思是“谈”,与介词to 或with 连用表示“与…交谈”,与介词about 或of连用表示“谈论…内容”.例如：
May I talk to you?
Tell 意思是“告诉”,表示告诉某人某事.例如：
Please tell us the good news.请告诉我们好消息.
本题强调的是说话的内容,因此选C