Who is your carrier? Have you resent the service books to the device and re-hosted your routing table?
I'm going to assume that since you switched from pay as you go to full contract service that there is something in the provisioning of the device that hasn't been applied. Re-sending your service books should resolve this. Go into your e-mail settings on the device, press menu, select "Service Books" then select to send service books. This should get you back in action.