It looks like you have to have gotten all the other Trust spells to date, plus completed all of Shantotto's quests, up to the Warp II spell. You do not, however, have to have completed ASA (I'm on the last fight but haven't finished it yet) to have gotten her. I went after picking up Ajido and all I got was the same couple of messages, but after swinging around to grab Volker and Trion, I came back and she gave me the cutscenes to get Shantotto.