Iphonef one of the speakers is a woman then she must be cautious of being soft-spoken. Indeed Allah, the Glorified, prohibited the wives of His Prophet, peace be upon him, the Mother of the Believers, may Allah be pleased with them, from being spoken to in a seductive manner by the one who is moved with desire. In the time of the prophet, aswell as the lifetime of the Companions, Allah be pleased with them, women were forbidden from being soft spoken, Allah (the Most High) says: {Do not be soft spoken unless the one in whose hear is a disease should be moved with desire but speak in an honorable manner.} (Soorat-ul-Ahzaab: 32)

So what about women besides them? Indeed their prohibition of being soft-spoken is from the most befitting. Therefore fear Allah O believing women! Do not be soft spoken; speak in a good and honorable manner. In other words, with no softness in tone nor with unnecessary prolonged speech. The Muslim woman must not address the men she is not married to freely, as she would address her husband.

The woman must be cautious in engaging in lengthy discussions with men that she is not married to, even if they are her relatives - those relatives that she is not allowed to be alone with according to the Divine Legislation.

She must also be cautious in raising her voice louder than normal and talking for a prolonged period of time and must cautious of showing beautification, gentleness, softness, elegance, and flirtatiousness in a gentle tone and soft spoken manner to a non-mahram.

Just as it is impermissible for her then it is also impermissible for the man to listen to her voice indulgently, even if she was reading the Qur'aan. If the woman perceives that then it is impermissible for her to continue speaking with him due to the trial that it leads to.

And here mention is set aside for the man, the protector of the home, to arrange his affairs based on guarding and upholding virtue and to protect those that are sacred. The woman of the home should not be the first one responsible for answering the phone while the male relatives are present. She should not respond in the presence of their absence under any circumstances, rather, the man of the home should direct her according to the circumstances and necessities he sees fit.

Then it is obligatory for her to hear and obey in honor those in charge of her, protecting what is appropriate of being protected and leaving off any issues of difficulty.

Source: Aadaab-ul-Haatif (The Islamic Manners of Using the Telephone), Pages 15 - 16.