What is the ruling on a relationship in which a man and a woman imagine they are having intercourse with each other over the phone until their desires are aroused?

Allah Almighty commanded lowering the gaze, avoiding a submissive tone of voice, and forbidding seclusion between non-mahram (definition) men and women; because these are means to temptation and pretexts for falling into indecency. Therefore, for a Muslim woman to display her charms to a non-mahram man is even more forbidden. The Prophet, peace and blessings be upon him, forbade a woman from describing another woman to her husband as if he were looking at her. Scholars explain this by saying that such a description causes temptation by captivating the heart. Therefore, it is advised to refrain from this action to avoid Allah's wrath, as it is a type of zina (fornication/adultery), even if its direct legal consequences do not apply.