Keep your original Google number
When you change your Google Voice number, your original number will remain active on your account for 90 days. Google Voice users who have paid to change their Google Voice number, signed up for Sprint Integration, or have paid to port their mobile number to Google Voice are eligible to keep both their old and new numbers on their account.
Once you've completed the number change process, both your old and new numbers will appear in the Phones tab for three months. At the end of the 90 day period we will email you with a list of contacts who are still using your original Google Voice number, so that you can update them.
However, if you wish to permanently keep your original Google Voice number as a secondary number you can do so for a one time fee of $20. All calls and text messages to your original Google number will still reach you. However, your primary Google number is the only one that will display on the caller ID when you make calls or send text messages. You can access this option by going to the Phones tab in your Voice Settings.