I suggest you look at your bill more closely and observe what the cycle dates say vs when you paid off the phone. A bill isn't going to reflect a sudden change if it already printed out.
We're glad to answer billing questions around your paid-off device agreement. Let's collect additional information. If your device was paid off yesterday, that would not appear on a current bill as it takes 4 weeks or more to generate the next statement. What is the date for the first bill that you're viewing? -Rita K.