I recently bought a set of MDR-1000X and this is how they behave. At work I’m sat next to the sales team, and without them I wouldn’t be able to do any work.
When they are on, but not playing music, I can still hear loud voices but it’s significantly dampened (like in the room next door).
With music playing, at a relatively low volume, I can’t hear anyone - unless someone is shouting right next to me.
I've found the opposite to be true. Got a pair of QC-35II that block everything but speech, which subjectively appears to become even louder, even when you listen to music.
They're great for airplanes, trains, and low-frequency background noise, but for a crowded office where people are having chats and telcos, I'd rather use Shure in-ear monitors.
It's a recent Sony top-model, which from reviews seems to be on the top together with Bose regarding NC technology at the moment.