We used to put a bell on Jasper, so when he was in the woods, we could always hear him.

Have you tried sardines? They are messy ,but I find the more smelly the treat the better response I get.

I would get a proper whistle, and when you are at home blow then feed, do it for about 2 weeks.

Jaspers recall is about 8 out of 10, I just put him on a lead well away from roads, ponds, squirrels and picnics