What is the ruling on a man speaking with a non-mahram woman on the phone in seclusion, continuously and for long conversations, even if it is for advice or to do good?

It is permissible for a man to speak with a non-mahram (definition) woman when there is a need, such as offering advice or performing good deeds, provided that Islamic guidelines are observed. However, one must be cautious about prolonged and continuous conversation, due to the intense temptation women pose to men, and out of fear of Satan who lies in wait for the son of Adam. Indeed, Mu'adh (may Allah be pleased with him) feared for his nation the temptation of women, and Sa'id ibn al-Musayyib mentioned that Satan approaches the son of Adam through women, and that nothing is more feared by him than women.