Before you start training recall the whistle the whistle (or any other sonic medium) the sound & the dogs >behaviour of coming back to you< should then be reinforced with short game, the game MUST be finished after a very short time & any attempts (expect many) by the dog to restart the short game MUST be ignored >under no circumstances< should the owner give in to manipulation attempts, just ignore. The prceedure needs to be carried out in as many diferent place (in the dogs perception)) as possible.
Only do this maybe once the first day & for only a short space of time, then maybe a couple of days later again in a different place with minimal/no other distractions to the dog - continue over a couple of weeks period without any pattern the dog can detect or predict. It would be far more important to carry this out 3 times only in a first 7 day period than 3 times a day over the same period.
The whistle used in the vids below is a Fox40 classic, the reason that's used instead of an ACME is because these seems a lot of ACME around & others can confuse a dog
This video at the first link gives a reasonable idea of going about it (conditioning the whistle as a positive reinforcer).
https://www.youtube.com/watch?v=8W3s3UfAgO4The second link shows the long term consequences of recall strength/value.
https://www.youtube.com/watch?v=R33iI12-N-U.