Does a wife owe expiation if she engages in a private phone conversation of a sexual nature with her traveling husband during Ramadan until she becomes aroused?

Spouses must avoid what arouses desire during fasting, because desire is among the things that one must fast from. The questioner must repent to Allah for violating the sanctity of the month of fasting and for exposing her fast to invalidation. If what she felt exiting was maniyy (sperm/vaginal discharge), then her fast is invalidated, and she must make up the day without expiation, other than repentance. However, if it was madhy (pre-seminal fluid), her fast is not invalidated. If she is unsure whether it is madhy or maniyy, the default is that the fast is valid and making up the day is not obligatory.