We're sorry to hear about that frustrating problem. Most often, this is caused by a lack of signal or a malfunctioning SIM card. We can activate a new SIM card for you at a T-Mobile retail store should that be the case.
Also, in case call forwarding was activated in error, you can dial # # 0 0 4 # (POUND POUND ZERO ZERO FOUR POUND) from your phone to cancel any automatic call forwarding.
Thanks for posting and have a great day.