I think phones got through the "back door". When the first cell phones came around, not many kids took them to school, but the only thing they could actually use them for was contacting their friends/parents during breaks, so there was no reason to ban them. A Gameboy was specifically an entertainment device, an old cell phone was more like a flashlight or a watch, something which could presumably be used as a toy, but usually was not.

Then, cell phones got cheaper, more kids got them, but they also got more advanced. In the feature phone era, a lot of kids had phones already, but the balance of genuine communication versus amusement was still leaning towards communication. This changed very rapidly, and at a time where phones in schools were already ubiquitous, though their use in class might not have been.