Before I give you my amazing list of awesome answers, I encourage you to get the numbers from the people you are actually calling. It can avoid this whole awkward, creepy part.
Here are some ways you can try to get around the question:
I found it in my phone and thought you put it in.
I was fighting a bear, and in return for sparing its life, it gave me your phone number.
I found it on the "most beautiful people on Facebook" Facebook page.
I borrowed "Captain Jack's compass," and it led me to a piece of paper with your number on it.
I asked Siri for the phone number of the most beautiful girl on Earth.
I found it on a piece of paper in my Lamborghini.
I saw the name "Angel" in my phone and decided to call it.
It was on my "missed calls" list, how did you get my number?
I know a guy.
I was going to come up with some impressive story, but I'll just tell it to you straight. My dad works for an agency that must not be named. He works on projects that I can't talk about but said that I could only have communications with one person. You're that person.