If you already have a Google Voice number you want to continue using as your Google Fiber Phone number, it's a simple process.
To add an existing Google Voice number to your Fiber plan:
Sign in to Fiber using the email and password you use for your Fiber account. Your Account page appears.
Under Plan Details, click Manage Plan.
In the Overview section, under Subscribe to Additional Services, click Add to Plan next to Phone.
Follow the on-screen instructions and select Use your current Google Voice number; then complete the Google Fiber Phone setup flow. Once completed, Google Fiber Phone will use your Google Voice number.
If you like, navigate to the Phone tab in Fiber to explore the Google Fiber Phone settings. You can start using your Google Fiber Phone number on your devices through Google Hangouts right away.