You're never really sure who is answering your message." - Ch. 4, The Student Newspaper Survival Guide by Rachele Kanigel, p. 33
This could be true with phone interviews if you've never met the person or if you just don't remember the sound of their voice. It can also be true of face-to-face interviews if the person is pretending to be someone they're not. No method of interviewing is full proof.
"You can't ask spontaneous questions... Reponses can be stilted or carefully crafted."
These are only important if the article your writing is particularly controversial or delicate in nature. Besides, you can ask follow-up questions and you can ask them in a way that tries to get an answer that isn't PR.