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