Unused NET10 minutes roll over to the next month. There is one important thing to consider. If you go past your expiration date without adding more airtime, then your service is canceled. That means you lose your number and you lose any remaining minutes in your account.
It's easy to keep the unused minutes simply by adding more airtime before the expiration date.
If you don't use up all your minutes, and you add additional minutes and airtime before expiration of your present airtime, the unused minutes will roll over. You never lose usused minutes as long as you add more before expiration. And the additional airtime is added to the end of your present prepaid period. So you don't lose prepaid airtime even if you add more early.
When you add minutes to your account, you are also extending the service time by adding the additional days of usage to the end of the present term. That means that if you buy more airtime well before expiration, you don't lose the remaining time. You keep that too. They simply tag on the extra time to the end of the present expiration date.
Remember, you don't lose your minutes unless you let it expire without adding more airtime.
NET10 has a solution for those who can't remember to add airtime. You can sign-up for a monthly service. For $15 a month they automatically extend your service for another 30 days and add another 150 minutes to whatever you have left. You never lose it and you can cancel this automatic renewal anytime. It is not a contract. It's always up to you if you want to keep it going.
I've been using NET10 cellular for several years now. Read my article about my experience switching to NET10