We're commonly asked if because of the merger between Sprint and T-Mobile that they will be able to use the Pinwheel device. However...
Sprint and T-Mobile merged financially, however they did not merge cellularly.
Meaning, if you have Sprint or T-Mobile you can utilize both networks but still will use your original network if you can.
Think about it like this, if you took the old T-Mobile coverage map and put it on top of the old Sprint coverage map you'd get an overlap. Anywhere that doesn't overlap but has T-Mobile, your phone would work splendidly, but anywhere that has ONLY Sprint or overlaps at all, it will not work (which is going to be pretty much everywhere).
If you want to use a Pinwheel device but don't want to get a T-Mobile prepaid plan, you can look into getting your Sprint account moved over to a T-Mobile plan.