Default Judgement awarded without proper service

I was just forwarded a copy of a garnishment interrogatory letter from my employer for a judgement that I had no knowledge about. I researched the case on the public records database and it shows that default judgment was entered after several attempts were made to serve me with the suit. All of the "serve attempts" listed in the record showed "Return to Sender." Furthermore, the debt is supposedly from AT&T Wireless, which I have always been a Verizon customer, from 2002. Besides having no knowledge of the debt, my understanding after some research is that the Federal SOL for cell phone bills is 2 years. The judgment was awarded in May, 2008. There is a hearing scheduled in two weeks, which I found out about through the public records database. The collection agency was Palisades. What are my options at this point?