Keep Your Original (Secondary) Google Voice Number Forever.

If you originally got an Google number and later on if you;

Changed Google Voice Number,
Ported You Home or Soho Office number to Google Voice,
Made your Sprint Number the Google Number,

Then you will have two Google numbers for 90 days. After the 90 day period, Google took back the original number.
But if you are attached to your original Google Number, Google Voice is allowing you to keep the number now! So your friends and family will still be able to text or call you on the original google number but when you call out or text, your new number will show up as the caller ID.
So if you would like to keep the original number, get to the settings tab, and click "make permanent" next to the number you want to keep. (There is a one time $20 administrative fee)
Go ring those numbers! and if you need further help, Google Voice Help Centercan help you.

Google Voice Blog: Make your secondary Google Voice number permanent

Google Voice without Google Number, your existing number

Google Voice has made it even easier to use by allowing your existing number to be your Google Number, Google Voice announced today.

As we have already told many of you, Google Voice and Google Number have made our lives and communications much easier to handle. We are able to to do call recording, call screening, getting text messages and even have transcribed Google voicemail messages as email.

So either way, with a Google Number or your own number, you will have ability to use many of the features that Google Voice offers, like the Google Voicemail.

There are many ways you can get an Google Voice account, one is to request an invite directly from Google or someone who already has a Google Voice account.

You can see how Google Voicemail makes life easier for many Google Voice users.

Official Google Blog: Use Google Voice with your existing number

Google Voice, With Your Existing Number, Mobile Or Landline

When I received my Google Voice Number from then GrandCentral, I was very happy because, I will have a number for life, no matter which carrier I use. I now realize that not everyone like to have a new phone number, even when provided with all the services that Google Voice offers.
Now you can use your own number with Google Voice, if you are willing to pass call forwarding, screening, and recording and gain much more than the your carrier offered, like transcribing Google Voicemail.
You can see for yourself the difference of using Google Voice number or your own number with Google Voice.

If you sign up for Google Voice with your existing number, you'll get:

• Online, searchable voicemail
• Free automated voicemail transcription
• Custom voicemail greetings for different callers
• Email and SMS notifications
• Low-priced international calling

But

If you decide to also get a new Google number, you'll get all of the above PLUS:

• One number that reaches you on all your phones
• SMS via email
• Call screening
• Listen In
• Call recording
• Conference calling
• Call blocking

The new feature, could used by people who already have a google number to ease with transition.

Google Voice Blog: Google Voice with your existing number
Google Blog also carries the news
Official Google Blog: Use Google Voice with your existing number