Mobile Share Value® customers will automatically get Rollover Data
on the network with the nation’s strongest LTE signal*
We’re proud to offer Rollover Data to our new and current Mobile Share Value customers. With Rollover Data, unused plan data rolls
over for one month. What’s more, if you’re on a Mobile Share Value plan, you don’t even have to do anything to get it—
Rollover Data is automatically part of your plan. For our Mobile Share Value customers, it's our way of letting you know how much we appreciate you!
This is from that AT&T site regaurding rollover data. It does state for ONE month not cumulative.