The Ruling on Speaking to a Woman on the Telephone
Question :
What is the ruling if a young, unmarried man spoke with a young unmarried woman on the telephone?
Answer:
It is not permissible to speak with an unrelated woman in a manner which may excite desire, such as by speaking words of love, flirting and subduing one's voice, whether on the telephone or otherwise. This is in accordance with the Words of Allah, Most High:
"then be not soft in speech, lest he in whose heart is a disease (of hypocrisy, or evil desire for adultery, etc.) should be moved with desire..."
As for speaking when there is a need, there is no objection to it, as long as it is free from corruption, but only in cases of necessity.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 5 Page 120