Can't guarantee anything, but call customer support to verify if the IRDB (Roaming Data Base) is up to date. It is periodically (usually automatically)sent to your phone that tells it which towers it has access to. Make sure you call from a home phone, not the cell phone.
Cingular "Raising the Bar" service has been horrible. I have never had service this bad from any carrier in 15 years. I am now locked into the 2 year contract from hell. Anybody have any advice?