That may not always be the case. When one of my best clients, Reuters Television, calls to see if I'm available for a program, the number on my phone reads, "unknown caller". I asked why and she said that since she's in an office with a few hundred other phone lines, they all get routed through the same source, or something, so they come up as "unknown".

So, at least for me, I always answer those calls to. To not do so would cost me thousands of dollars!