What is the Islamic ruling on a married woman having phone sex with a man who is not her husband, and is that considered zina (adultery/fornication), and what should she do to atone for her sin, and is she obligated to inform her husband after her repentance?

Praise be to Allah, who has blessed this girl with repentance. As long as she has repented from this sin, which is establishing a relationship with a foreign man and engaging in phone sex, then Allah Almighty accepts the repentance of those who turn to Him if they abandon the sin, regret having committed it, and resolve not to return to it. It is better for her not to inform her husband about what she did, because Allah Almighty loves for a servant to conceal their own faults. What she did is not fornication that necessitates a prescribed punishment, but rather it is from the "fornication of the tongue and the ear," and its expiation is repentance.