Delay or no?
 

Team A wins rally. Coach A yells "Sub". R2 sounds the whistle to recognize substitution request. Legal libero replacement follows. No player comes to sub into the game.

The coach obviously did not intend to make a substitution request. He was telling his players to make the libero replacement. However, the official blew the whistle.

Delay because he said "Sub", or inadvertent whistle because he didn't actually mean "Sub"?

I went with inadvertent whistle. Opinions?

He legally requested a sub, and it was acknowledged. No sub = delay by rule.

Now, if you to go with inadvertent whistle, make sure that coach knows to never yell sub again unless he wants a sub.