NC headphones aren't designed to block all noise, only repetitive noise, basically white noise. Other noises, like ringtones, actually are louder or easier to hear on many models of NC headphones, as is speech around you. Noise-cancellation's purpose is to attenuate white noise, such as the drone of your office A/C, or your airplane's engines. It's not meant to block anything else, and if your headphones do, it's because they're also acoustically isolating (e.g., they have bin "cans" over your ears physically blocking noise, but that has nothing to do with NC).