Everybody takes part in the roast, tries their best to embarrass Michael except Phyllis, who doesn't take part in the roast. I wonder why?
It's really not Phyllis's personality to openly roast someone. In fact, I can only recall two instances of Phyllis blatantly insulting Michael throughout the entire series --
One, when Michael interviews for corporate (S03E23),
PHYLLIS: I think it's gonna be Michael.
OSCAR: Do you really think he's qualified for that job?
PHYLLIS: No, but he wasn't qualified for the job he has now and he got that one.
and two, when Michael gets counseling from Toby (S07E02):
PHYLLIS: I'm glad Michael's getting help. He has a lot of issues, and he's stupid.
It is worth noting though that when Michael announces the roast to the office, Phyllis does immediately start writing down material for the roast, however, once they actually get to the roast Phyllis doesn't have any papers/note cards at the table with her (whereas everyone else who roasted Michael did).
If I had to guess, I'd say that Phyllis did intend on roasting Michael but then Bob Vance talked her out of it.
Additionally, Darryl Did roast him but Michael did not give a reply to Darryl either. I wonder why (?)
Simple!, because Darryl wasn't in the office at the time of Michael roasting everyone. Also, Michael is afraid of Darryl so I doubt he would have said anything anyway.