Call forwarding is a carrier specific feature that is widely supported by most carriers. It is often enabled and disabled by calling specific carrier codes on the phone itself.
Light Service Plan Call Forwarding
TURNING ON CALL FORWARDING:
Open the phone's diapad, and enter *21* followed by the 10 digit phone number to forward calls to, then press #. Place the call and wait at least three seconds, you should hear a confirmation tone before ending the call (NOTE: you may not hear this at all)
Example: *21*1234567890# forwards your calls to the phone number 123-456-7890.
TURN OFF CALL FORWARDING:
Dial #21# and place the call. Wait at least three seconds to hear a confirmation tone before ending the call (NOTE: again, you may not hear this at all)
Call Forwarding with Other Carriers
Please refer to your carrier for the exact steps to turn ON and OFF call forwarding with your provider.
Here are links to the largest US carriers specific instructions for setting up call forwarding:
T-Mobile Call Forwarding FAQ (under Calling Services)