T-Mobile Launches 'Data Stash' Program
Following today's “Uncarrier” event, T-Mobile has announced something new called "Data Stash". This allows you to roll over unused data to the next month. While most carriers just restart your data plan, this carries over an unlimited amount of unused data every month for the next 12 months. After 12 months it restarts.
Each user has their own Data Stash amount so if there are multiple people on one account it doesn't matter how much others use. However, this also means that you can't transfer your stash.
This is available to all current and future Simple Choice customers with a plan over 1GB. It also excludes people with unlimited data, of course. Also as an incentive they're offering everyone 10GB of rollover data just for being a T-Mobile customer.
This will start with your first billing cycle of 2015. You can learn more about Data Stash here.