all you have to do is dial the number to your voicemail...
its the same as the number on the call forwarding screen on the phone options.
dial the number and save it to the speed dial in the number 1 spot... my speed dial entry for the number 1 position reads like this:
"Voicemail+18056377255"
i did that once when i wiped it, and i had to reassign it also.. that worked for me, but i was pissed before i figured it out. not as mad as when i made my BB black and white with nonrefreshing screens and wire mesh icons.. i could never get to the themes screen before the screen was all black.. i was so chapped.
good luck.