Oh they can enforce it just fine. Some guy in UK tried 3D printing a gun and saw what’s what, granted he called support which is stupid as hell, but everyone has a non-zero chance of slipping up. And once you do, I hope you don’t have unfinished business outside. Impracticality of enforcement only becomes an issue if it’s ridiculously widespread, which this is not.

Given that your phone will see far more active use and will have to be connected to some kind of network, the chances of it getting detected and landing you in trouble is quite high. You have a better chance with getting out of there if possible or getting rid of your smartphone if not.

On a side tangen I don’t buy into the “anyone can build a gun” argument anyway. You have to make a frame, barrel, blackpowder, cast lead… it’s a long hard process that will result in an underpowered / impractical gun or most likely both, something 99% of people won’t bother ever bother with due to the sheer friction involved if nothing else. That argument holds weight with knives (sharp stuff in general), sheperd’s slings, maybe a brass knuckle even but not really with guns.