This is a bugle call played by military forces in the UK and elsewhere to call the troops to the canteen for meals. For that reason it has been popularised with the lyrics "Come to the cookhouse door, boys, come to the cookhouse door.". See the links at the left to download the sheet music or audio files. More bugle calls are listed in our article about the history and cultural impact of Bugle Calls
The image below is the first page of the sheet music, and here you can download the full sheet music for Meal Time (bugle call) in PDF format.