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No extra charge EVER, unless you have money in your wallet!
If you use all your data in your plan, you just don't have data service until next billing cycle.
Chizz :))

You'll be blocked data access. When you go to internet, you can't go anywhere regardless you see FIZZ LTE on top of your phone. As soon as you went to next billing cycle, you can access internet.

Don't forget this service is PREPAID. You pay for what you want ahead of time and then service stops after you reach the limit (except for unlimited options). If you put money in your wallet, it is assumed that your wish to use that to pay for overages and will be charged as such.