Chanel

The distraction of the park would probably be too much for the poor dogs :D Ideally you start the whistle training with no distractions and then add distractions gradually. The park would probably be the final step in the program because everything there would be more interesting to the dogs than a bell or whistle.

Benson I've also trained my dogs to recall at the sound of a bell and a bicycle horn so you can use just about anything. To get Benson interested ring the bell when he's close & can see you have the ball. When he comes you reward him with a game of fetch. It may take a couple of weeks to motivate him but make sure that he only gets access to the ball when you use the bell or whistle.

Pheeble you can use almost any whistle. I've used a cheap soccer referee whistle and an expensive hunters duck whistle with the same results. You can also substitute a squeaky toy or anything that makes a noise. For the first few weeks I use the whistle (or substitute) to signal mealtimes and I also add cuddles/praise when I'm phasing out the food rewards.