Good points. Most of us do have food available any time they are awake and several of us have food available even when they are asleep.
When I train my dogs I use treats. One of them is so crazy over food that I could use her regular dog food but treats work for both. You don't get guarantees with living animals, about anything could happen, but treats are a standard way of training chickens too. It usually works.
The idea is to get then used to receiving something they really like when you call. That means be consistent, if you call them have a treat ready. A call could be a certain yell (mine is "here chicky chick"), shaking a bucket with treats, a bell, a lot of things. But it needs to be something that only happens when they get treats. Will it work on every chicken on the planet? No, probably not. But it will work on most of them.