What is the ruling of the Shari'ah on phone sex with one's husband during the daytime in Ramadan while fasting, and how can atonement be made for this sin?

What transpired between you and your husband could fall under two scenarios:

1. If ejaculation did not occur, the fast is valid, but the act is reprehensible (makrooh). 2. If ejaculation occurred merely by talking: The act is forbidden (haram), and repentance is obligatory. The fast is valid according to the Hanafis, Shafi'is, and Hanbalis. The fast is invalid according to the Malikis.

However, if the conversation was accompanied by foreplay that led to ejaculation, then the fast is invalid, and you must make up for it.