No expert here, but maybe combine the timer with a new recall/vocal command (like comign to you and barking)? So teach the recall with a new word, then when that is solid, start adding the buzzer on the timer when you say your recal word, slowly eliminate the word?
No idea, but that might be how I went about it if I didn't go to a trainer or anything.
Commander Charly Titan (GSD)-15 Jan 2010
Lady Athena (BB)-2011-2012 (RIP baby girl)