Well the one sure fire way is to be supportive of Isabelle during the first day "Intro Quest Set". Thus far, or at least from my testing, as long as you act like a decent human being Isabelle should be giving you her number regardless of whether or not you reach out to her in the Cafeteria. Just be supportive of her and help Lindsey out when she needs it and you are golden. There are probably other points at which you can get the number further down the line if you try to romance her but I'm still learning and haven't found them yet.