If I pay $20.00 per month and don't use the minutes...can I carry the credit balance over to the next month or do I lose it?
$15 will go to your plan. With the $5 you have left, you should have 14 minutes for local calls. If you don't use your minutes before the specified period, your minutes will expire. But if you top-up before the expiration date, the remaining minutes will have the same expiration as your new top-up.