What is the ruling on sexual practices over the phone with one's fiancé, claiming that it is permissible because the families are aware of the engagement, and is it valid to marry him after this request?

The questioner should know that this young man is a stranger to her, so it is not permissible for there to be any friendship between them. What he requested from her is not lawful except for a husband. Allah Almighty has forbidden merely softening the speech. He says: "O wives of the Prophet, you are not like anyone among women. If you fear Allah, then do not be soft in speech [to men], lest he in whose heart is disease should covet, but speak with appropriate speech."

In summary: what that man requested is not permissible for him or for her under any circumstances, and a suitor remains a stranger until the marriage contract is concluded.