Sony Ericsson S710a

I won one of these from the whole Train & Gain contest and love it. However, I'm having some text messaging problems and apparently employees are the last people to get tech support on devices.

All of my other phones (nokia's and 1 motorola) work great with text messaging on Cingular. The s710 with the exact same sim card is having some major difficulties and I'm wondering if I'm the only one or if anybody has some suggestions.

It says message sent, but most people never actually get the message. And I only get roughly 1/5 of the messages sent to me. My service center # is +19703769301. It was working for the first few days, but lately just junk.

Any suggestions would be greatly appreciated.