Careful about using the T-bone headset (P/N 258260).
Whilst it conforms to TSO-C139, the amount of regenerative feedback during transmission makes the transmission sound like coming from the depths of a well (lots of echoing, etc.), particularly on the B744 and A330.
The amount of feedback is sufficiently bad (subjective qualification, but not by me) to the point that our company has specifically banned the use of the T-bone. A very expensive white elephant for those that purchased it without taking it for a test drive.
As a precursor to purchase: as the B757 is IFR certified, the headset must conform to TSO-C57a/C58a or TSO-C139 (which has superceded TSO-C57/C58 WEF 05 AUG 05).