What I already knew is that masturbation is Haram (sinful), even if the Qur'an doesn't clearly state it, but I found something very interesting, written by Ibn Qayyim al-Jawziyya: that it is allowed, only if that man only if that man has no wife and fears the he will somehow suffer without it. However, he still considers it Haraam.
However, even if solo masturbation is a debated subject, phone sex is a type of sexual relation between 2 people who are not married, which is specifically mentioned as forbidden by the Qur'an.
The conclusion is that phone sex is OK only if you don't masturbate and do it with your wife (or husband). :)