I think the difference is pitch; the phone is at a pitch that is more audible to the human ear than the voice through whatever construction material there is. OR the phone is never actually answered...
Could the answer to this be the same reason why when I'm in a bar bathroom near where my group of friends is sitting, there are certain voices in my various groups that I can always hear loud and clear and others that I cannot, even if they are all shouting?
I think so; it's probably the same reason why we could always hear Paul talking down in Donner no matter where we were.
Commenting by HaloScan
