T-Mobile previously announced Data Stash for postpaid plans only, but now the un-carrier is bringing the same feature to prepaid customers too.
For those un-familiar with T-Mobile's Data Stash, it allows you carry over any unused LTE data to the next month. Put simply, you'll be able to store your unused allowance for future use. Data can be kept on your T-Mobile account for up to a year before the system starts to clear data saved from 12 months prior.
Simple Choice prepaid customers will be able to take advantage of Data Stash from March 22. Everyone eligible will receive a 10GB sum of data, and once that's used up they'll begin rolling forward any unused data at no extra charge. See the announcement post linked below for more details.