Greetings and thanks for coming to our support site. That's no bueno the phone has a crack but I'd like to share some info about the warranty. Unfortunately, T-Mobile isn't able to accept devices with damage under our Exchange a device under warranty program. Did you happen to opt for some kind of handset protection when you first bought the phone? You could try reaching out to Samsung to see if they have options for repairing the phone.
Manage your T-Mobile account