This is the traditional bugle call known as First Call, which is used by military forces to instruct the buglers to assemble. I guess the first bugler who gives the call needs to use a reliable alarm clock! The call is familiar now from its use in the U.S. at horse races. 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 First Call (bugle call) in PDF format.