You can pay any amount you want above what you actually owe. Your account will then be in credit, and what you owe will be deducted from this credit.
Edit: For example, my first monthly bill is $50 voice + $30 data = $80 total. I send a check for $410 dollars (to cover this month's voice and a whole year of data). My account now has a positive balance of $410-$80 = $330
Next month, I get the $80 bill again, and I send them a check for $50 (this month's voice). My account balance is now $330+$50-$80 = $300. And so on.