Pipelines PM here. The model is a bit different, it's a pay as you go per blocks of 1,000 minutes. So if you normally use an extra 2,800 minutes per month you'll pay $30/month usually, but if for some reasons youd don't use it as much and drop to 1,500 extra minutes the next month you'll pay $20 for that month instead of $30/month every month of the year. For this reason we don't carry over minutes month to month.
Sten