Welcome to the community, Fred!
IndieGoGo use pledge as a terminology instead of a purchase, as they have projects on their site that collect contributions that may not be related to a rendered service/product. In the case of BraX3 project, pledge equals a purchase similar to a pre-order.
T-Mobile is preferred telco provider for the BraX3. You can find more info on telco compatibility in this post: BraX3 Telco Compatibility – United States
You should be able to transfer your current phone number to TMO. Best to consult with them directly.