You won't see the number. Even if you were to somehow get a hold of one of their phones it wouldn't be in the call history. The reason why is follows the tune of seven children is so that easier to remember and they can immediately delete their call history.

Besides, even here that number isn't up for investigation. I don't know what Haibara meant when she called it Pandora's box, but from what I've seen them do it's better not to risk it-yet.