Haptics. On phones that support it, the die gives a gentle buzz when it lands and a rattle when you shake. iPhone web pages can't vibrate, so they stay silent there.
Shake to roll. Give your phone a firm shake to throw the die, like rattling it in a cup. iOS asks for Motion & Orientation access the first time — tap Allow (or use the on-screen Enable nudge). No motion sensors? The Roll button always works.