To help you out, here's an excerpt from their website. There's more there on their "Shrinkage Plan".
What is Shrinkage?All Monthly Unlimited plans include SHRINKAGE and unlimited nationwide talk, text, web, email & 411. SHRINKAGE reduces your payment by $5/month for every six on-time payments, up to $15/month in total. Monthly Unlimited shrinks to as low as $35/month, Android Monthly Unlimited to as low as $40/month or $45/month for BlackBerry Monthly Unlimited.On-time PaymentsOn-time payments are made on or before the monthly payment date. Your monthly payment date remains the same every month as long as you successfully make your monthly payment. However, if you make a late payment, your payment date will be reset to the day of the month before the date your service is restored (e.g., if your service is restored on February 15th, your monthly payment date will be reset to March 14th).
Knowing the terms of your plan would be a good idea.