Be prepared to answer the phone, even when you've just started a new job. Answering questions with things like, 'I don't know; I just started working here' or 'This is my first day, so, um, if you can hold on I'll see if I can find out' are terrible responses, both for you and for your new company. It's important to realize that when you talk in a professional capacity to other people you are directly influencing your brand and your employer's brand. When you use the just-started-working-here excuse up front you come across as trying to lower expectations to accommodate a subpar performance. That's not the direction you want to go.
The direction you want to go is either, 1] anticipate and prepare for questions by doing some research in advance so you will actually know the answer, or 2] simply say something like, "Of course. Give me a minute and I'll be right back to you," and then move heaven and earth to find out the answer asap.