Google officially released incoming number portability for Google Voice. If this is a feature you have sorely missed for the past few years, you may now jump for joy if you so wish. Here are some great instructions from BGR,
Please note that it costs $20 bucks to port your number. Also, feel free to share in the forums how this will greatly enrich your life, or generally just be pretty useful.To port your number into your Google Voice account, simply click on Settings in the top right corner, and then Voice settings. Now, on the phone tab next to your Google Voice number, click on Change / Port. Now click on “I want to use my existing mobile number instead” and follow the on-screen instructions to port your number.
Source: BGR