A typical cycling jersey will have back pockets designed specifically to prevent things from falling out: the top edge of the pockets spans the entire width of the back and contains a band of elastic material inside. As you put your jersey on, the stretching action of that band is what keeps the top edge tight against your back, preventing things from falling out. I.e. even if you put some thicker items inside your back pockets, the pockets will still remain closed. Pockets like that are very reliable when it comes to preventing things from falling out.

Note though that if the top elastic band of your jersey pockets somehow breaks or gets worn/stretched out with time, the pockets might become a lot less reliable, especially for holding relatively heavy slippery items (like cell phones). Once your back pockets begin to flap instead of staying closed, you can no longer trust them.