Do unused minutes rollover?

The Base plan (the plan you start your month with) is valid for 30 days and unused balances from it will not roll over. However, unused minutes, texts, and data from Top Ups do roll over to your next billing cycle. The way they roll over depends on the network you are on.

On the  GSM LTE Network, minutes, texts, and data from Top Ups are valid for 30 days from the date they were added to your line, regardless of when the base plan ends. Keep in mind that you do need an active line  (with a base plan) to have service and be able to use the unused minutes, texts, or data from Top Ups. 
On  Super LTE, minutes, texts, and data from Top Ups roll over and have their remaining amount added to the next month's base plan, provided the next plan has been paid for in advance or you have auto pay set up already, and the new plan or auto pay includes a base plan of the same category as the top-up.