My guess is, if the answer of your question is within the phone like "what is the date today" she will reply by speaking. If the answer is not in your iPhone, and is not on Internet, she will reply by speak as well like when you ask her "how old are you?" Or "tell me a story please"
But if the answer of you question is on internet, but not within the iPhone like "tell me about the number PHI" she will pull up a webpage instead of speaking out the answer, certainly that's how Siri is programmed to operate at this date as she doesn't have all the world info into her data base as for yet. So she will just perform a search on the internet like you would do if there was no Siri. Now, I think she could still read out whatever she pull from the web as an answer to my question. Only apple knows why she wouldn't read it out. But I hear Siri will grow up in iOS 9 for better serve you.
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