I got into the Google Voice
preview program 3 weeks ago. Been playing with it since, and now I'm not sure how I was able to use my phones without it.
I took the plan where Google assigns you a new number. I've been migrating all the people who call me over to this number. One thing this does, is allow both my mobile and my home land-line to ring at the same time when this number is dialed. Which ever one I pick up gets the call and the other stops ringing. This is like multiple extensions of a land-line, but they are in fact different numbers. You can even press * on a call to your G.Voice number, and have it transfered to the other line right in the middle of talking. Say you took the call on your land-line, but are headed out somewhere, but want to keep talking. Press the *, transfer the call to your mobile number, it'll ring, pick up the mobile, and then hang up the home phone.
I've also set my mobile to forward to my Google assigned number when busy, not answered, or out of service. This makes my home phone ring when I miss a call on my mobile. I just now got the "no-answer forward" set up on my home phone too, so it does the opposite, missed calls at home go to my mobile via the Google Voice service.
The first time a caller hits the Google number (either by dialing directly, or being forwarded in one of the cases above), they are asked to say their name. This is configurable, and if you have someone in your Google Contact list, that also makes them considered to be known. This information is used for when you pick up the call if you have call presentation enabled. Remember how the old answering machines worked in you could hear what the person was saying, and pick up the call if you really did want to talk to them? When a call comes in on Google Voice, and you answer it, you hear the caller's name announced (either from their recording or a computer speaking the name from the Contact list). You're told to press 1 to pick up the call (in the mean time the caller just hears the phone ringing). You can also send them to voice mail. This is where it's neat, you can hear them recording the voice mail, and interrupt to actually take the call if you changed your mind.
Voice mails are actually transcribed with a speech-to-text system, and can be forwarded to your e-mail or texted to your mobile. They can then be picked up by dialing your own Google number from one of your associated lines, or any other number and then pressing star. Though the neat thing is you can go to the Voice website, to read the transcription, or listen to the message with the inbuilt Flash player.
You can mark a caller as spam, and when the hit the system (direct call or forwarded) they'll go straight to voicemail. There's a stronger "block" which plays the "doo-dee-deep" sound and informs the caller the number they have dialed is out of service.
Text messages to your number are forwarded to any associated mobile phone, and saved online in an inbox-like system. Texts can be sent from the web interface also. Actually, calls can be placed from there too. What happens is your phone rings, you pick it up, and are told that the call to the number you typed into the website is in progress. The other person's phone is being rung at that time. My mother also has the service now, and is using it to call my brother in Africa for only $0.19 a minute, as opposed to the $1.79 she was paying before. There's a way to use this assisted dialing straight from the phone too, by dialing your Google number, and following the prompts.
That brings me to the cost. This service is FREE, other than the internationally dialing, which you pre-pay for (with your Google Checkout account), all the other features I've mentioned have no charge. Google's plan is to take the content of the text transcripts and display related AdWords ads when you're viewing the inbox, much like how Gmail works now.