—Sir,      called you just now. I told him to phone again 20 minutes later.
—OK. Thanks, Nancy.

首先明确选项中每个不定代词的意思someone某人；nobody没有人；anyone任何人；everyone每个人；结合选项联系I told him to phone again 20 minutes later．可以推断句子的句意是“刚才有人给你打电话，我告诉他20分钟后再打来。”，所以不定代词someone在本句中符合句意，所以答案选A。

略