One idea might be to use the old thin clear tube headsets from Plantronics, they would be very cheap and effective for close work in call centers.
Mike Sandman,
www.sandman.com , can do most anything with handsets, I would call him at 630-980-7895.
Also try Refurb Supplies to see if they can do anything for you, they have a ton of stuff and will work with you on most anything.
Southern Telecomn still has the Roanwell Confidencers which were and still are the Gold Standard for noise cancelling but as far as I know they only fit the "G" style round receiver and transmitter style.
If a standard carbon type handset will work on an iDCS phone (don't have a carbon handset here to try-sorry) then you could get the round handsets with Roanwells in them and use a headset button to be able to eliminate usage of the hookswitch.