You will never get charged hidden data overage fees with Public Mobile. Since they are prepaid only, if you use up your data, they'll send you a message telling you you've reached your limit and offer you to add on more data. They will actually send you a notice that you've reach 75% of your data limit so you can be conscious and prevent running out of data completely. If you choose not to add on data you just won't have any data until your next billing cycle stops. This is no big deal as you can find wifi everywhere! To prevent using up your data before your next billing cycle, I recommend choosing a plan that meets your data use needs. Check with your current provider to see what your average usage has been over the past few months to get an idea. It's also a good idea to turn off data to all apps that you don't need to be accessing with data. Sometimes they use data when you're not even using the app which is a total waste!