The British have been hailing motorized (motorised) taxis since 1897, so one could rightly assume they know how a thing or two about how it’s done.

London Taxi Whistle (No. G1427). Hail a cab or sound the alarm with this two-tone whistle, from a company that has been crafting them since 1870. Nickle-plated brass. 4 x in. Made in England.