Speaking trumpets were used by officers and engineers at fires dating back to the beginning of the 19th century. These trumpets were used to amplify their voices in order to communicate orders at the scene of a fire. Officers were often recognized because they had the horn hanging around their necks.
The symbol of the trumpet was later used to denote rank on insignias.
As a side note, the Speaking trumpet was also referred to as a plunger, referencing a notable resemblance to an outhouse plunger.