Hi Robert,

Noise canceling headphones work by blocking out a repetitive, predictable sound, like the humming of an engine, or similar constant sounds. They don't block out new sounds, like an unexpected car horn, or a phone ringing.

If you want to block out all sounds, you could get isolation headphones, and then you'd still be able to speak into a mic hooked up to them.

Have Fun!
Peter Gannon
PG Music Inc.