He Wants to Kill Her Or Fuck Her

The word master is mostly used by workers and with a family like Zoldycks, it was a high chance that the man on the phone was their butler, because not all ordinary servants would have such direct access to contacting Ford Zoldyck.

"Hmm… go on." Ford was humming mostly, not saying much just listening. It was probably his attempt to keep their conversation private from prying ears.

What does he think that she would eavesdrop on someone? She would never! 

Well, unless it was Ford Zoldyck.

Admitting our faults to ourselves is a one good way of feeling liberated. And when Moon felt liberated after admitting that, yes she was indeed listening to his private conversation with his butler, she found honing in on the butlers voice thru the speaker became much easier and clearer now.