I've seen it written down (so it must be true) that participating trumpeters never actually sounded the charge - principally because the motion of the horse made it all but impossible to maintain the call, although retaining his teeth on the inside of his head must also have had some appeal - and the command was invariably delivered by the CO with any/all of word of mouth, flourishing of the sabre, and rapid movements of the head to make the plume on his headgear shake noticeably (watch the King's Troop commander at Trooping the Colour).