Most headphones I've tried simply don't do well with phone calls when I'm in a noisy place, like New York City. I can usually hear the other person just fine, especially if the headphones I'm using have noise cancelling. But the problem is that the other person can't hear my voice. Instead, they hear the city streets, and some of the people I've spoken to say "It sounds like you're in a washing machine," or "Are you in a hurricane?"
That's when the headphones come off my ears, and I switch the call to the phone itself rather than the headphones.
With the NC 700, Bose has managed to cancel out a large portion of the noise around me for the person I'm talking to. In especially noisy places like the Financial District in NYC, the noise cancellation isn't total, but the person I'm speaking with can at least hear my voice clearly over the din.
Even in noisy open office spaces, the chatter of nearby colleagues barely transmits over to the person I talk to during a phone call. I took a call with someone else who wore the Bose NC 700 in the crowded Miami Airport, and it sounded like he was in a living room with maybe a couple people on the other side having a politely quiet conversation instead of an airport full of people and loud announcements.
This is why you'd want the Bose NC 700: if you want a better experience of taking phone calls without taking off your headphones. Otherwise, if that doesn't matter too much to you, you'll be fine with the QC 35 II.