From the horse’s mouth
After T-Mobile had done it with their Data Stash plan, one of the other US carriers has opted in to the idea of data plans that allow you to use last month’s unused data allowance. Here, old AT&T had provided the Rollover Data feature to their new and existing Mobile Share Value customers no matter the plan they currently reside on.
But they don’t have the same kind of option that T-Mobile has where you could “harbour” a large quantity of unused data allowance to do something like a huge download or video-stream session. Instead you can only use last month’s surplus after you use up this month’s allowance.
What I see of this is that those who offer bandwidth on an allowance-based model could dabble in the idea of an “unused allowance rollover” scheme. Here, the providers could allow one to harbour a significant amount of unused data or simply limiting this allowance to last month’s surplus.