Is a phone call between an engaged couple that includes sexual topics, and they are close to their wedding, forbidden or permissible?

The fiancée is a stranger to the suitor. It is not permissible to shake hands with her, nor to see her, nor to speak with her except out of necessity and in the presence of one of her mahrams. Speaking with her about sex is absolutely forbidden. The suitor must abandon this type of talk and repent to Allah, because the marriage might not materialize, and then his knowledge of her secrets would be forbidden. Let the suitor occupy his heart with the remembrance of Allah, for "Verily, in the remembrance of Allah do hearts find rest."