Instructions to use Google Voice as your voicemail when people call your cell phone number

Enable Call Forwarding For Google Voice On My G1 and T-Mobile

The Important Parts For Me:
*004*1[GVnumber]*11#
Note: the GV number is the 10 digit number (that "1" right before it is the "1" we use to dial long distance, so do don't enter it twice).

And to disable if I want to go back to T-Mobile's Voice Mail...:
##004#

Full Directions For All Phones And Carriers:
Instructions to use Google Voice as your voicemail when people call your cell phone number - Google Voice Help