Is speaking about sexual matters over the phone, while imagining it, considered zina (fornication/adultery), and what is its expiation?

Telephonic sexual conversation between spouses is permissible under certain conditions: that no one overhears them, and that both are secure from falling into forbidden acts such as masturbation after the call.

As for sexual conversation with someone other than one's wife, it is forbidden. The one who engages in it risks being punished through their own family, as Imam Al-Shafi'i said: "Be chaste, and your women will be chaste in the حرم (sacred precincts)..." The Messenger of Allah, peace and blessings be upon him, called it zina (fornication/adultery), saying: "The zina of the eyes is looking, the zina of the ears is listening, and the zina of the tongue is speaking..." Its expiation is repentance to Allah Almighty.