Reuters
Here's there press release:
To kick off Un-carrier 5.0, T-Mobile today introduced "T-Mobile Test Drive" and pioneered a new way to buy wireless in the U.S. With T-Mobile Test Drive, anyone can get an iPhone 5s and unlimited nationwide service to take T-Mobile's network for a data-intensive, seven-day spin at no cost.
Starting June 23, people can sign up for T-Mobile Test Drive at www.t-mobile.com/testdrive. A few days later, they'll receive an iPhone 5s and they can put T-Mobile's data-strong network to the test for seven full days.