Is it permissible for a Muslim man to speak with his fiancée over the phone while he is fasting?

All praise is due to Allah.

There are specific guidelines for speaking with one's fiancée, as detailed in question number 7274. If these guidelines are met, particularly regarding the safety from desire and the presence of a need, then it is permissible. If any of these conditions are not met, then it becomes religiously forbidden. In the case of fasting, the sin is even greater because it negatively affects the fast by diminishing its reward, and a fasting person must preserve their fast.