Even the voice is Purda

يَا نِسَاءَ النَّبِيِّ لَسْتُنَّ كَأَحَدٍ مِنَ النِّسَاءِ إِنِ اتَّقَيْتُنَّ فَلَا تَخْضَعْنَ بِالْقَوْلِ فَيَطْمَعَ الَّذِي فِي قَلْبِهِ مَرَضٌ وَقُلْنَ قَوْلًا مَعْرُوفًا


“O Wives of the Nabi! You are not like any other women. If you fear Allah, then be not soft of speech (when conversing with strange men at times of dire necessity), lest he in whose heart is a disease (spiritual corruption) aspire (for you)”. (Surah Al-Ahzab)

The following are the regulations to be deduced from this verse: 
1. At times of dire necessity, it is permissible to converse with a strange male from behind a veil. 

2. This conversation, although permissible, must be conducted deliberately in a firm tone. It is not permissible for a woman to speak in soft, alluring tones with a male, even from behind a veil. (A point of note for girls’ Madrasas, their students and teachers).

3. If this is the regulation for women, the same regulation will apply to men to a greater extent.

4. The pervert that derives pleasure and satisfaction from listening to the voice of a strange woman, has been declared as spiritually ill in the words of the Qur’aan Majeed. 

5. Those men who appoint females as receptionists, secretaries, telephonists, etc. should scrutinize their intentions for doing so and take special heed to this verse.

(Effective remedies for spiritual remedies - Hazrat Shah Hakeem Muhammad Akhtar Saab Rahimahullah)