John.
This source states an officer with a flag and a trumpet call were used.
I have looked at a few on line lists of trumpet/bugle calls and can not see a call for a truce listed and again if there was, it would have to be internationally agreed. So perhaps it was the combination of an officer with any available flag [excluding the regimental colours I assume] and a random trumpet call [so as not to be confused with the transmission of an order], that was used to get the other sides attention, that became the accepted signal.
Paul