What is the ruling on a husband caressing his undelivered wife by phone during the engagement period, which led to the husband ejaculating, and what is the expiation for that, knowing the difficulty of determining the time of imsak in his place of residence?

If ejaculation occurred before Fajr, the fast is valid. If it occurred after Fajr, you only need to make up that day, as you did not intentionally violate the sanctity of Ramadan (definition). As for speaking with your fiancée, if she is legally your wife, then all permissible speech is allowed. Otherwise, it is not permissible to speak about matters that arouse desire, and masturbation is absolutely forbidden.