"What you don't use, you don't lose," Legere said during a live interview that was streamed on the carrier's website. The initiative, he said, is "screamingly obvious."
Eligible customers will be enrolled in the program automatically, Legere said. To kick things off, T-Mobile is also throwing in 10GB of free data.
Data Stash is the latest "Uncarrier" move rolled out by the nation's No. 4 wireless carrier as it aims to take on industry giants Verizon and AT&T.