Praise be to Allah.
We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, who answered as follows:
“There is nothing wrong with that. Yes, it is permissible.”
Even if there is use of the hand?
“There are some reservations about use of the hand (i.e., masturbation). It is not permissible unless one fears that one may commit zina otherwise.”
So as long as there is no use of the hand, it is OK?
“Yes, as long as there is no use of the hand, it is OK. He can imagine that he is with her and there is nothing wrong with that.”
They should also make sure that no one is listening to what they say or is spying on them. And Allaah knows best.