Call diverting is a network service. You can change them by sending the network shortcodes from your phone but this can be fiddly unless you are familiar with the codes. Simplest in my opinion is to contact t-monile exact what you want and they will set it up for you.
13-06-2013 8:34 - edited 13-06-2013 8:46
If you want to extend the time before a divert activates, you can add **[time in seconds] after the number. So to activate divert for all calls to number 12345 after 30 seconds you would enter: **61*12345**30# CALL.
In addition to that, the picture below is an overview of some often used Call Divert options:
The codes mentioned are what most operators use since this call forwarding feature is a service from your network operator as what cjlim mentioned, but they can be different so double check with T-Mobile.
Note: If these codes are not supported by your network you will get 'Unknown request' or 'Request denied' from the network as a response.
Hope this helps.