There appeared to him (E.K.) four very fair castles, standing in the four parts of the world: out of which he heard the sound of a trumpet. Out of every gate then issued one trumpeter, whose trumpets were of strange form, wreathed, and growing bigger and bigger toward the end.
However, we were never told who the Trumpeters are. It's assumed that they aren't really that important in terms of practice, more a formality for announcing the arrival of the King and his entourage.
The following site suggests a solution:
Scroll down to Trumpeters. I disagree with them. Attached is a high-level document of the O.'.S.'.D.'.L.'. that shows my thoughts on the Trumpeters.
I can't recall the exact quotations that specify that the Trumpeters have a trumpet with 6 holes in it (which is why the central two letters are chosen, for there are 6 openings from there, leading into the names of the 6 Seniors). There are a few other quotes which initially formed my opinion, so if/when I find them I'll share them here to support my views.