The advice basically was whether it was an offence at all to hack into a voice-mail box if there were no unheard messages within. Yates rather astonishingly says he was told that the hacking per se was not illegal unless the hacker listened to messages that the owner themself hadn't yet heard.
So, the prosecution would have to prove that there were unheard messages, and since that could be difficult, a prosecution could not be guaranteed.
Now, I'm not a lawyer, but I rather feel that this is one of those cases where the court would rather assume that the hacker had, in getting access, had the opportunity to listen to messages, and that it would then be for the hacker to prove that he did not hear any fresh messages, rather than for the prosecution to prove that he did.
In other words, if Yates had been doing his best to avoid having to prosecute, this is the sort of argument he might have used to justify such avoidance.
I do rather think that someone, somewhere, should have written something down.
Oh dearie me, I think I need more popcorn.