Is the fast valid for one from whom fluid was discharged after a phone call with her fiancé that included flirting about her body, and she uttered exciting words, then she felt desire and imagined intercourse during the following day?

You both must repent to Allah Almighty and not overstep the bounds of Sharia. Therefore, do not speak to each other except out of necessity and in a place safe from temptation. Your fast is valid, whether the fluid discharged from you was maniyy (semen) or madhiy (pre-seminal fluid), as long as it occurred at night. It is most likely madhiy, which is impure and necessitates wudu' (ablution). If you are unsure whether the discharged fluid is maniyy or madhiy, you may choose [which ruling to apply], and ghusl (ritual bath) is not obligatory unless you are certain it is maniyy. If you are afflicted with waswasah (obsessive whispering), you must strive against it and turn away from it. Ghusl for janabah (state of impurity requiring ghusl) is not a condition for the validity of fasting, and the discharge of maniyy as a result of thinking does not affect the validity of the fast. The fatwa we follow is that the discharge of madhiy due to direct contact or similar actions does not invalidate the fast.