Does interaction between a man and a woman via phone, like that of spouses, with the impossibility of marriage and the existence of a strong love relationship, constitute zina (fornication/adultery)? And how can one expiate this sin and stop it, especially with the addiction to masturbation?

Love between young men and women is Islamically impermissible, and acquaintance with the intent of marriage is a gateway to corruption. Your conversation with a man over the phone is not zina (fornication/adultery) that necessitates the hadd punishment, but it is a reprehensible evil and a path to immorality. The Prophet, peace and blessings be upon him, called it the zina of the eyes, ears, tongue, hands, and feet. Your duty is to hasten to repent by severing the relationship with the man, occupying yourself with what benefits you, strengthening your connection with Allah, choosing righteous company, and engaging frequently in dhikr (remembrance of Allah) and du'a (supplication).