I do actually have some noise-blocking earphones (passive, not active canceling) and they work very well. However I've missed phone calls due to this. For example, my supervisor calling from a meeting to ask a question relevant to the meeting, in a room without a computer (so email wasn't feasible). I've also had several instances of people talking to me but not noticing that I have earphones in my ears and can't hear a word they're saying.

So as much as I like them, they're a little problematic. If I had my own office the interruptions wouldn't be as much of an issue.