What is the ruling regarding a husband divorcing his wife over the phone by saying, "You are divorced from me by three [divorces] until the Day of Judgment," after an argument between them?

The majority of scholars, including the four Madhhabs, hold the view that a triple divorce uttered in a single phrase counts as three divorces. However, Shaykh al-Islam Ibn Taymiyyah and those who agree with him believe that it counts as a single divorce. The occurrence of divorce does not differ whether it is uttered face-to-face or over the phone.

Based on this, if your niece's husband divorced her with three pronouncements, then according to the majority of scholars, she becomes unlawful for him and is not permissible for him until she marries another husband in a desired marriage, consummates it with him, and then he divorces her. As for the view of Shaykh al-Islam Ibn Taymiyyah, it counts as only one divorce, and he has the right to take her back before her 'iddah (waiting period) ends, provided this divorce does not complete the three. The preponderant view is that of the majority of scholars. The phrase "until the Day of Judgment" does not affect the ruling of the divorce.