Google Voice phone numbers are locked by default and must be unlocked before they can be ported to OnSIP. The Google Voice website offers some basic guidelines to make the move easier. Of course, the most important thing to note is that Google Voice, unlike most carriers, charges a $3 unlocking fee. While this is a bit of a nuisance, we have found very few problems when attempting to port from Google Voice to us after the number has been unlocked.
Once your number has been unlocked, you can follow the normal porting procedures as found in our Knowledgebase. In place of a phone bill, however, you can log into your Google Voice account and capture/print information that includes the billing name, billing address and phone number to be ported. Most online portals have a billing section with your address information for credit card billing. Along with the paperwork containing your name and address, also provide the unlocking-fee receipt in order to ensure this process is completed as quickly as possible.
Once your phone number has successfully ported and is working as desired, it's always good practice to notify the losing carrier (in this case, Google Voice) and inform them that you've ported your number out. You should also request that they remove your number from their system. Otherwise, it is possible that current customers of the losing carrier will be unable to reach you at your new destination.
Updated July 2015