Is it haram to talk to someone of the opposite sex over the phone? If so, where I can find this answer in the Quran or a hadith so I can tell and show it is haram.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Talking with the opposite sex without any necessity will lead to prohibited things like flirting, eroticism etc. No doubt this is forbidden in Islam. Unchaste people always like to search illegal ways to satisfy their bad desires. Almighty Allah Says (what means): {O wives of the Prophet, you are not like anyone among women. If you fear Allah, then do not be soft in speech [to men], lest he in whose heart is a disease should be moved with desire but speak with appropriate speech.} [Quran 33:32]

A beautiful lady could very easily invite men to herself. Therefore, Allah says: {lest he in whose heart is a disease should be moved with desire.}, i.e. hypocrisy or evil desire for adultery, etc. as the interpreters of the Quran have explained.

Allah Says in another verse (what means): {And let them (believing women) not stamp their feet so as to reveal what they hide of their adornment. And all of you beg Allah to forgive you all, O believers, that you may be successful.} [Quran 24:31]

If the stamping of feet can excite men’s evil desire, what do you think about a phone conversation between men and women? No doubt this is a huge temptation, every Muslim should keep away from it.

Allah knows best.