"First call" is a bugle call with three distinct meanings.
At a horse race, where the tune is also known as the Call to the Post, it is a signal that all mounts should be at the starting gate because the race is about to begin. The tune is usually sounded by a bugler five to 10 minutes before the scheduled start time of the race. The call serves a similar purpose in dog racing.
At a U.S. military installation it is a pre-reveille "courtesy" signal, sounded around 05:50, originally to assemble the trumpeteers to deliver the reveille that would be forthcoming at 06:00. Some locations also sound it a few minutes before retreat (lowering the flag at the end of the day).
In other military contexts it may be used (e.g. 5 minutes) prior to sounding Assembly for any particular formation. Individuals who have not fallen in by the last note of Assembly may be considered late or AWOL.
On ships of the US Navy, First Call is sounded at 0755, 5 minutes ahead of Morning Colors (raising the national ensign), and 5 minutes before Evening Colors (lowering the national ensign). In the absence of a bugle, the word is passed, "First call, first call to colors." The same ceremony takes place on shore establishments but not on ships underway.
First call is a result that is the first to appear on a list for a search query. First call is used sometimes to measure a site's popularity, for example, searching on Google for the subject and verifying the site's position on a list, if it is the first.