So in Season 8 - Episode 23 "Turf War", Robert had a drunk night. He apparently sent an "inappropriate" voicemail to Nellie as she says:
Nellie: I got your voicemail. From – from last night.
Nellie: And the answer… is yes, yes, yes, yes, and never.
... But Robert has no clue as to what he said, so he turns to Pam to find out.
Robert: Apparently, I left a phone message for Nellie last night, and I need you to find out what I said.
Pam: Um, I am a little busy.
Robert: Yes, ‘course. Why don’t you list the things that would keep you from helping me.
Pam: Yeah, I can make you a list.
Robert: Let’s do it now. What’s number one?
Pam: Why don’t I help you now?
Robert: There we go.
So Pam steals her phone and shows to Robert but as the messages were so disturbing, she deleted the voice messages and we never got to see what was the voicemail sent to Nellie.
Later, Nellie tells Pam about the voicemail and how it is wildly inappropriate:
Pam: No. My goodness. You have a lot going on. With Robert and everything.
Nellie: Oh, god, Pam. Don’t get me started.
Pam: No, I will not.
Nellie: You’ve just got me started. Robert… is… a filthy beast. I mean, don’t you get the feeling, he’s just thinking of fifteen different ways to do you?
Pam: Well –
Nellie: I mean, the man talks of nothing but sex.
Pam: But sometimes he talks about flesh… and bacchanals.
Nellie: I cannot even tell you what he left on my phone last night.
What was it? How bad could it be? What does "yes, yes, yes, yes, and never" refer to?