It was by no means watertight -- many (most?) kids had a "burner" phone that was dead or broken or whatever that they turned in (because nobody would believe a highschooler who said they didn't have their phone with them lol) and they kept their real phone. But at least the expectation was set that you can absolutely not ever be seen using a phone in class.
Overall I think it was a good policy. I don't think it's workable at large schools.
In NYC at least you simply can't tell families that kids have to leave phones at home -- they are an essential tool for safety and travel, especially for younger kids. My 12 year old can take the subway to school by himself, but ONLY if he has a phone in case he ends up in some wackadoodle situation where his train has gone express or is on another train line and he needs to call his lifeline (me!) to figure out how to work it out. But they do not need to have them in the classroom.