AT&T will let 50M customers carry over unused data to the next month

It's only the previous month of unused data from that month's allotment (and that data gets used first), but that's better than nothing, I guess.

Assuming 15GB plan:

Month 1 (15, 0 avail): Use 10 GB, roll forward 5

Month 2 (15, 5 avail): Use 8, roll 7, lose the previously rolled 5

Month 3 (15, 7 avail): Use 20, roll nothing (lose the unused 2)

Month 4 (15, 0 avail), etc. etc.

So this saves you if you have one outlier month, but it's still not as good as rolling what I've nominally paid for.