
I have a hunting breed too, and they can be selectively deaf off lead as are many breeds and individual dogs on occasion.
The only thing you can do if you want your dog to have some freedom is to stack the odds in your favour.
Choose places that are least likely to get your pup in danger or be a nuisance if she does ignore you and bog off. Take her out hungry. Walk off he first burst of energy so that she is a little tired when you do let her off, and try to choose a time and place with minimal distraction as already said.
Teaching reliable recall at home and on a long line first is important, but sooner or later you need to bite the bullet.
If she is toy orientated all the better. Perhaps have a lure on a piece of line to throw so that you can real in for her to chase back to you, Fetch if she plays it, and of course really yummy teats that she gets at no other time. something like baked liver cut into tiny pieces (it's rather rich) as most dogs will sell their souls for it.
Only call her when she is looking your way or already heading in your direction. If she looks like she might head too far away, get all excited jump up and down run in the opposite direction all excited, so that you are really exciting.