Why isn't my unlocked sidekick ID working?

[heather]

I recently purchased an unlocked sidekick ID off of eBay. I've been told by many people that I would be able to make phone calls and send and recieve text messages ONLY. I know that I cannont access any other data such as web browsing. I am currently using an AT&T SIM card in the sidekick and I can only make phone calls. However, when I go to send a text message, I recieve an error that reads: SIM card not ready. I've left it alone for awile to see if it would eventually be ready but nothing happens. Can someone please tell me how to fix this? ;(