How can a man's conversation with a woman over the phone or through chat be unlawful if it adheres to general etiquette and does not lead to what is unlawful, and would they be sinful for it according to Sharia?

The relationship between men and women who are not mahram, under the guise of friendship and brotherhood, is a gateway to fitna (temptation/strife) and corruption. Claims of good intentions should not be accepted, for Allah the Almighty addressed the elite of men and women saying: "And when you ask [his wives] for something, ask them from behind a partition. That is purer for your hearts and their hearts." This indicates that mixing and removing partitions is not purer for hearts. Scholars have stipulated that it is impermissible to speak with a young woman who is not mahram without need, because it is a potential source of fitna. The fact that some relationships do not lead to haraam (prohibited acts) does not negate the ruling of Sharia, which forbade them because they are a potential cause for falling into haraam. The Sharia ruling is based on the apparent and regulated legal cause (illah), which is the probability of benefit, not on the wisdom (hikmah), which is the benefit itself. Therefore, these relationships must be severed to guard against Satan and his steps.