Return
Even if you don't have a warranty, you might be able to talk them into replacing it. Most states have adopted the Uniform Commercial Code (UCC) which is a collection of statutes relating to buying and selling goods. It says that a "non-conforming" good may be returned within a "reasonable time." The buyer can get his money back, store credit, or a replacement AT HIS DISCRETION. A "reasonable time" is usually defined by the law of the state you're in, but you could just make the argument that it's "reasonable" to expect your $400 phone to work for more than a few months. It's worth a shot.
Yes, I am a lawyer.