Unless your leaving your phone in the attic, it shouldn't get dusty.... If you are using it constantly, dust cant accumulate. If you keep it in your pocket, check them, they might just have a lot of dust and junk in them. Haven't experienced this myself.
It does, and it's one of the reasons I returned, along with some of the same said dust that apparently made it's way inside the camera housing. Here are pics of test I did (and posted) with same case (Speck Presidio Sport), both phones starting out clean, same pants pocket, 10 mins each. Plus a pic of the particle under the camera lens cover, which was NOT there when bought just 4 days ago. Before anyone says it's the white face of the 7+ making the screen look better, I'm ONLY looking at the black part--the actual screen. The X definitely has a different static charge, and much of the dust particles are much finer than that which sticks to the 7+:
Just FYI...I've had every iPhone since the 4, have had screen protectors only on two, rest are native, keep them in my front blue jean pocket, clean them at times (DO have microfiber & usually use just damp, sometimes with safe eyeglasses cleaner), but, typically I have only gotten smears on my screen. But, with the X, it's been REALLY dusty, like there's a static charge or something. Wondering on the dust inside the camera area, if it got sucked in from elsewhere inside (not sure there's an opening to suck dust in via...).