Please unlock nokia 6010

IMEI: 010700003124511
Firmware Version: V 8.62

Nokia: 6010
IMEI: 010700003124511 (No spaces, no slashes)
Network: T-Mobile To Go
Country: USA

Can you please provide me with the code to unlock this? My mother bought this phone about a year ago and just recently bought another $25 card as she didnt have the money. When you turn it on now it says "Sim card registration failed" with Quit. When I quit the phone functions perfectly, but you cannot make outgoing calls and obviously she no longer has a working tmobile phone number to call customer support.

I think this might be the only thing for me to do?

Any help will be appreciated. Thanks