I can't tell you how many times I've heard someone goof up the telling of an interview question. Sometimes they leave out a vital piece of information, other times they just talk too fast or give away too much information. If you are doing a phone screen, you can add a lack of body language and phone volume to the list of communication challenges.
It's also hard to give examples verbally. Try saying "an array of integers with the values 56, 425, -65 and 371" out loud. Even if you do manage to rehearse the question and example perfectly, the candidate will probably ask you to repeat it so they can write it down.
Technical interview questions are better expressed in writing.
Some might say that poorly explained interview questions are a good method of determining how well the interviewee can ask follow up questions and communicate. I disagree. Making yourself look stupid shouldn't be a part of your standard interview process. By doing so, you will only convince the candidate that your company would not be a good place to work. Remember, the candidate isn't the only one being judged here.
I wouldn't go so far as to say that your questions must always be typed up and printed out, especially not for the in person interview. However, the sheer act of typing them up will force your team to really think through the questions and be less likely to pull them out of thin air. For phone interviews, I would suggest that most of your questions should be typed out. You'll still be able to evaluate the interviewee's communication skills as they discuss their solution and thought process.
Take some of the guesswork and uncertainty out of your interview process by typing out your interview questions.