Port your Google Voice number out to another service provider
To port your number out of Google Voice, unlock the number. Then, let your service provider know you want to port a number to their service. They do the rest.

Unlocking your Google Voice number costs $3, but it's free if your number was originally ported in from another service provider.

On your computer, go to the Google Voice unlock page
Point to the Google Voice number you want to port, and click Unlock my number.
Click Continue.
Pay with your Google Account. You’ll get your receipt by email.
Let us know that your number is unlocked. 
Note: If your mobile service provider asks for your account number and PIN, provide your ten-digit Google Voice phone number and the PIN you use to access your Google Voice voicemail.
Note: Without a phone number, your account will change to Google Voice Lite.