Disclaimer: i do not own a blackberry. i don't know if this will work. (i used WM/Fring/SipSorcery.com(currently closed to new accounts)) this is based on info from this forum
Step 1- open a sipgate.com free account
Step 2-install a sip client (for BB-
http://www.blackvoib.com/Download/BlackVoib.jad , for Windows Mobile/Iphone use Fring.com)on your phone
Step 3- set up your sip client with your sipgate settings
Step 4- log in to google voice and put your sipgate phone number as your forwarding number
Step 5- dial from google voice website. (you will need to dial from your browser for every call. until sipsorcery reopens)
this should route the entire call through web/wifi with no 'minutes'. report back if it works