A couple of points regarding the calls.
the best success I have had, was on ground where there was a reasonable population of deer but not what i would describe as a large population. Where I stalk now there are large numbers and my success with the call is poor to be honest, compared to elsewhere I have used it. I think that when there are a lot of stags in the area whistling away, if they were to check each other out all the time they would simply be doing that every few minutes! I think when there is large numbers they almost switch off to the challenge or inquisitiveness typically displayed by stags on ground with less of a population.
Aother point, I have used the call as late as December (out of season licence), well out of the rut, to bring stags into a shot.
I also learned a few years ago, and it was discussed on here, that you can spook younger less confident animals with it so don't use it on immature beasts too soon...
It is also a good idea to give it a go if you have a beast running away from you after being spooked. I can't recall the number of times I have watched one being bumped, then stopping for a quick look back at the whistle I am now letting him hear!!!