This is a brief explanation of how this method is introduced effectively; be patient, it is worth the investment of your time.
Using a whistle. The whistle will give you an additional tool to use with your dog’s re-call; it is an as well as, not an instead of. The whistle can never lie to her, if you blow it, you must give a food treat and praise for the response.
Starting. When giving her main meals, prepare her food and as you place the bowl on to the floor and your dog is about to eat, blow the whistle 3 times, short, but loud blasts. This will start to build up the association of the sound of the whistle meaning “ah food”.
As well as the food times, during the day, (3 sessions spread out during the day, 5 or 6 “practices” per session), when you are very close to your dog as in right next to you and you have her absolute attention and a treat in your hand, (always using high value such as cheese, hot-dog or chopped, cooked chicken), say her name and blow the whistle, peep, peep, peep and immediately give her the treat, followed by a sincere, “good girl”. (Note; there is no obedience involved in this; it is not necessary for her to be in a sit, you need to reward/praise the reaction to the whistle).
Note. Do not rush to implement this when out on a walk or to test the theory, it takes a lot of patience. If you transfer the sound of the whistle to outside to soon, you will almost certainly train her to ignore it and therefore lose the positive effect it may have. I suggest that you use an Acme gun dog whistle, tuned and pitched for your specific breed of dog available on line, or any good pet shop. (Buy more than one, they are easily lost and/or forgotten).
Progressing this exercise. After a week or so, try calling her when she is at the other end of the hallway in line of sight, then out of sight after a week or so of this, try with her out of sight in the house. Initially it should be from the next room building up the distance as you progress. When she responds 100% of the time try when she is in your sight but in the garden and you are inside. When out take her to a contained dog walking area and if there are no dogs present, repeat the line of sight process.
Keep practising at this level for a while, then, start to increase the distance and the distractions. Do not set her up to fail. Blow the whistle when out on a walk and she is next to you on the lead, this will help to increase her response when there are distractions such as people, traffic noise etc.
If you find that your dog stops responding be prepared to go back 2 stages and start from there. Don’t keep blowing the whistle in the hope that you will get a response, all you will do is start to training your dog to ignore the sound and lose all future benefit.