White flags, trumpeters, drummers, etc. = it was not just trumpeters (though those seem to have been preferred):
“In Felde sendet man einen Trompeter od. Tambour … ab, der, wenn ihn der Feind auch nicht annimmt, doch unverletzlich ist. Ein Trompeter begleitet auch die Parlementärs; in Ermanglung eines Trompeters geben sie sich durch Wehen mit einem weißen Tuche zu erkennen.”
Universal-Lexikon der Gegenwart und Vergangenheit oder neuestes ...,usw., Volume 16, pp. 458–59
https://books.google.com/books?id=t4eMcz4RWYEC&pg=PA458&dq=parlementaire+Trompeter+Waffenstillstand+Offiziere&hl=en&sa=X&ved=2ahUKEwidoL3TvIvkAhXYGDQIHf0RDiQQ6AEwAHoECAAQAg#v=onepage&q=parlementaire%20Trompeter%20Waffenstillstand%20Offiziere&f=false
The Aust Dienst–Reglements mention protocols for receiving an enemy parlementaire (Susan is right about this word, by the way), but do not describe what specific trumpet calls or drum signals were recognized as indicating a desire to parlay. Mostly these are injunctions against talking to the enemy envoy beyond the minimum and ensuring his blindfold is securely fastened. See:
https://books.google.com/books?id=kt2gAAAAMAAJ&pg=PA14&dq=Parlament%C3%A4r+reglement+%C3%B6sterreich+Trompeter&hl=en&sa=X&ved=2ahUKEwjM77yp05LkAhUq0FkKHTYSCKkQ6AEwAHoECAEQAg#v=onepage&q=Parlament%C3%A4r%20reglement%20%C3%B6sterreich%20Trompeter&f=false
https://books.google.com/books?id=aRJ1NsMoGXEC&pg=PA245&dq=Parlament%C3%A4r+reglement+%C3%B6sterreich+Trompeter&hl=en&sa=X&ved=2ahUKEwjM77yp05LkAhUq0FkKHTYSCKkQ6AEwAXoECAIQAg#v=onepage&q=Parlament%C3%A4r%20reglement%20%C3%B6sterreich%20Trompeter&f=false
This (Item 194) highlights another language problem between French and German (English too) when describing an "armistice" or '"ceasefire" or "truce" or other temporary halt to hostilities.
https://books.googleusercontent.com/books/content?req=AKW5QaeymBoTd-zdx5_4HVFms2RVTPnfQ6XVY6Rjc8BI8RJlY9YbisIFH2faRKjydBqnc53r_cU796bw7iw2ueBJNUzA6UtDq4_nBNKaLjd0GR8DAz3JdqxIBotYIBR0Ef1-F43F3qQxlI0qTSDkFNWd30cZfBI1rj6Fe-I5EDdiT97J1ZjjGWhEWMYjwdj-D42XhvOpuWdcgPfZZo7mzTw390oLS04ig_p66g_Cs9jUJkyn4n3lFq94FrCyFSs5RwmRsM6L9p6CidljsdJptyljxo4ejFUJtaPEaW5EFbJ41rYqxqToFjc
And this found by accident-- how to compose your next essay in the Austrian military writing style!
https://books.googleusercontent.com/books/content?req=AKW5Qad8-_TQpuaHf9m9UK3iDmyVqblVBfssiC3lkRoZ3P27pR7Dg6kE0cmkAus5YOx6vZtCIWS7f1_EMKMhiwovqwrvusluTwSPkfR4NyNJKhD6P4AU6IpmqNl2YYxDdDl6RLLGK2E8vDHuMA6I46s0Wx7KwRX-0na2m4Mipfm9F4RiKeusvXyY-ZwEepsGs_aymB4qMLq8Ogu9oabRd1MP7W1TkGqJUvp5cmJ4mzBFd7yzOccIKtDp5k7ow6BT7ssX6fi-SDk4dPVM6MN3GKkGAQ1N3u5cP1ZyWTZKhKTzeFxldB0QrDc