"Thank you for calling Old Navy at Jefferson Mall! This is Kristin, how can I help you?"
Like, honestly. It's extremely easy for you to say, "Oh, I'm sorry, I have the wrong number."
And if you just say that, you get the added benefit of me telling you to have a nice day! Who doesn't like that?
Just say something. It only takes a few seconds and eliminates a lot of awkwardness.
Along these lines, if you call someone and they don't answer, leave a message. It's just polite. It's awkward to see a missed call on your cell phone and not know who it was or whether you should call back or not.