You can get rollover data every month, but your rollover doesn't accumulate.
I have 31 GB of available data this month. I've used 14 GB. My normal plan is for 20 GB. If the month ended today, I would get 26 GB next month.
This is the last I repeat. All you're saying is that you get rollover each month that you don't use all of your allotment. Meaning that you're not using the rollover.
If you use any of your rollover, the next month, you don't get any rollover because you would need to go through your allotment before you can use rollover.
If they used the rollover first, then you could
use the rollover each month (provided you don't go over your rollover and allotment). For example:
1st month with rollover (using your 20 gb a month allotment) with you having the 31 gbs to use:
20 allotment
11 rollover
31 total
To use any of the rollover, you have to go through your 20gb allotment.
Even if you only use 20.1 gbs, you get no rollover for the next month.
If they used the rollover first, you could
use rollover each month:
11 rollover
20 allotment
31 gbs total
Here, you could use 20.1 gbs and still have almost 11 gbs to use the next month.
This is lame and ATT knows it. Actually, it's pretty clever on their part but it's still lame.