We put a similar question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him): Is it permissible for the spouses to speak about sex on the phone and provoke one another until one or both of them reaches climax (without using the hand, as that is haraam)? This is because the husband is always travelling or they only see each other every four months. 

He replied:

There is nothing wrong with it; yes, it is permissible. 

Question: Even if that is using the hand? 

Answer: Using the hand is subject to further discussion. It is only permissible if the person fears that he may fall into zina. 

Question: Without using the hand there is objection to it? 

Answer: Yes, without using the hand there is no objection to it, and if he imagines that he is with her, there is nothing wrong with that. End quote. 

But the danger with that is that others may spy on these meetings via the internet. 

This action also promotes doing haraam things such as masturbation etc. So in order to be on the safe side it is better not to do that, because the Prophet (peace and blessings of Allaah be upon him) said: “Sins are the private pasture of Allaah. Whoever lets his flock graze around this pasture will soon transgress into it.” Narrated by al-Bukhaari (1946). 

And Allaah knows best.