Hi Cashis,
It sounds like your device is provisioned incorrectly. Is this a new device or have you had it for a while and this trouble just started?
If you just upgraded to the 8100 it's not uncommon for service providers to fail to properly provision you over to a data plan, etc., or your device hasn't received the proper service books.
Furthermore, if you purchased the device second-hand (such as on e-bay, etc.) you must notify your carrier of your device's new IMEI number... it must match your SIM or all kinds of weird things can happen.
Hope this helps, let me know the outcome.
Thanks