I have purchased a Blackberry through a 3rd party retailer before and when I had warranty issues, I called T-Mobile. They are super nice, they'll walk you through some basic troubleshooting stuff and ask a bunch of questions, but if it's a warranty issue, they will send you a new phone and get you to swap out your SIM and send the old one back and I believe only charge you for the shipping.
Unless it was under like 7 or 14 days and you're looking for Amazon to give you a full refund.