Learn German: Auf Wiederhören, Frau Klein!

Auf Wiederhören is used specifically when you are ending a phone call. While sehen means to see, hören means to hear. So literally, you are saying until we hear each other again rather than until we see each other again.

Why is Wiederhören capitalized? Isn't hören a verb?

You are right that hören (to hear) is a verb. However, German allows you to turn verbs into nouns, which means they must be capitalized. Here, das Wiederhören acts as a noun meaning the act of hearing again.

Does Frau mean Mrs. or woman here?

In this context before a name, Frau is used as a formal title meaning Ms. or Mrs. While the word by itself does mean woman or wife, German uses it directly as the standard polite title for adult women, regardless of their marital status.