Quote from smarts
Agreed its prolly got something to do with the fact that anko still has that curse mark. Anyways Orochimaru was sealed away not killed. Sasuke has prolly already figured out how Itachi sealed him and where he would be.
Ehm im quite sure his main body is stuck in a genjutsu like world after being cut by the sword of totsuka inside the sword itself, so my guess is that he can be contacted through his residual chakra in Anko's seal, even his ashes/leftover sheddings have life in them or maybe it is more correct to call it parasitic effect, so it is not hard to believe his chakra can be used to contact his consciousness stubbornly living in that genjutsu world. Unless Zetsu was wrong on its effect then thats my best guess.