Source: http://lifehacker.com/5810249/you-can-now-keep-two-google-voice-numbers-for-a-price
Plenty of people have been porting phone numbers to Google Voice lately, but part of that process has always been saying farewell to your old Voice number, and sending it back to Google for recycling. That process allowed 90 days more use of the old number so you could get everyone to use the new one, but now you can extend that period to the realm of forever by just paying $20 to make the old number permanent. Your caller ID will show your new number whether you ported it to Google or just had it changed from an old one, but friends and family who refuse to update their address books will still be able to reach you at the old number indefinitely.
Make your secondary Google Voice number permanent | Google Voice Blog
