In a discussion the other day, the topic of "first call" came up.  In the 8 yrs I have been in the hobby, usually before we form up there is a first call given before the fall in.  About 5 - 15 minutes before.  

Does anyone know where this comes from?  Is this in a drill manual?  Is this possibly a reenactorism?

I know there were bugle/drum calls for assembly and such (http://www.geocities.com/cwfifedrum/schedule.html), but I don't think I have seen anything that specifies a warning prior to the assembly.