Is talking about sex between a young man and a young woman over the phone or internet considered zina (unlawful sexual intercourse), and if so, what is its expiation?

It is forbidden to talk about sex with a non-mahram woman over the phone or internet, or similar means. This is considered the "fornication of the tongue," as stated in the Hadith: "Indeed, Allah has decreed for the son of Adam his share of fornication, which he will inevitably attain. The fornication of the eyes is looking, the fornication of the tongue is speaking, and the soul wishes and desires, and the private parts either confirm or deny that." This is called figurative fornication, and its expiation is sincere repentance to Allah.