Acme Thunderer Dog Whislte 210.5
A fabulous whistle for training dogs at distance and to come when called. Robust and great intensity of sound. Used by the Allsorts team for many years and wouldn't use anything else.
Pop it on a lanyard, or keyring so it attached to you. Most people buy two in case they lose one or for another member of their family.
Or get a different pitch (as denoted by the number e.g 211.5) to train a dog to a different pitched whistle.