have you tried using orchestra earplugs in addition to the headset? They block out a lot of extraneous sound but still allow you to hear important conversation. Might be worth a try if you can't find the right headset.
I use Dragon speech activated software, earphones cover ears and has mic, but the mic picks up background noise so not ideal in an open office.
You could try a product that I used as a drummer called 'ProPlug'. They're really designed for swimming/snorkelling but are made of clear silicone rubber, shaped and fitted ear plug with a tiny hole in the centre.
They take out a lot of the frequency's that cause hearing damage and also seemed to take away a lot of background noise yet left me able to hear normal conversations. Most importantly they're comfortable to wear.
They come in different sized and it's important that they are sized correctly. The are unobtrusive and fairly cheap.
The more expensive option would be noise cancelling full both ear headsets but they can be very expensive and isolateling.
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