What should a fiancé do who still has two years before marriage and suffers from intense lust, and is it permissible for him to engage in intimacy with his fiancée over the phone as an alternative to masturbation?

It is not permissible for you to practice masturbation or to speak with your fiancée about what you mentioned. She is still a stranger to you, and the engagement does not make forbidden things permissible. It is only permissible to look at the fiancée to the extent that encourages marriage, and to speak about matters related to arranging the marriage out of necessity only. Fear Allah and hasten to marry to protect your chastity, or increase in fasting, for it suppresses desire.