No, leftover minutes from your base plan do not carry over to your next billing cycle. However, all add ons carry a 30 day validity from date of purchase.
How does Auto Pay work?
What payment methods does US Mobile accept?
What do I do if I run out of talk, text, or data in the middle of the month?
What taxes and fees will I have to pay with US Mobile?
Can I change my plan in the middle of the month?
What happens if I don’t add funds to my plan within 45 days of my billing cycle