You can do it the standard way with * codes for services that support anonymous calling or with Google Voice you can do something like I did for anonymous calling.
Caller ID Block (Anonymous Calling)
Caller ID Block allows you to mask your name and number information from appearing on the phone you are calling. To use Caller ID Block for one call only, dial *67 and then the destination number. To use Caller ID Block on a persistent basis, dial *81 from the handset attached to the OBi. All calls will use the Caller ID Block feature until you cancel the Caller ID Block. To cancel Caller ID Block, dial *82 from the handset attached to the OBi.
Here is what I do.
http://www.obitalk.com/forum/index.php?topic=5451.0
