Dial:

*21*[dest]#

Where [dest] is the full phone number of your voicemail system. For T-mobile US it's 1-805-637-7243 (1-805-MESSAGE). So you would dial:

*21*18056377243#

You can also do this by activating all call forwarding from within the menus in the phone app on your Blackberry.

On Blackberries, either of the above methods should result in a little "all calls forwarded" icon on your home screen that looks like a phone handset with an arrow.