I am so happy with Google Voice! If you haven't tried it out, you may want to! First, it's free!
Here's what it does! You pick one number from Google Voice with your area code. What's neat is that you can have Google Voice search for a word you like, a special combination of numbers, your first name, or just any combo and it will let you know if a number is available with that combination. Then, you control what happens when someone calls that number. For instance, if you are waiting for an important call from your spouse, you can have your mobile and your phone at work both ring when your spouse calls so you don't miss it! You can have separate answering messages for each of your contacts. You can screen calls or have them go straight through or a mixture of both. You can have a certain contact get a notice that the line is disconnected...helpful for those pesky calls and you don't even have to think about it! There are apps for your phones, too! The newly updated Droid app works great and there's one for Blackberry and iPhone. So, when you make a call from your phone, the phone number shown is your Google Voice number. Google voice also gives you voicemail by text and my email. There's so much available, you'll just have to take a look at http://voice.google.com.
Here's a short video for Google Voice