As Google search revolutionised the Web, and Gmail revolutionised free e-mail, then one thing’s for sure: Google Voice, unveiled on Thursday, will revolutionise telephones. It unifies your phone numbers, transcribes your voice mail, blocks telemarketers and elevates text messages to first-class communication citizens. And that’s just the warm-up.

Google Voice began life in 2005 as something called GrandCentral. It was, in its own way, revolutionary. It was intended to solve the headaches of having more than one phone number (home, work, cellphone and so on): Having to check multiple answering machines. Missing calls when people try to reach you on your cell when you’re at home (or the other way around). Sending around e-mail at work that says, “On Thursday from 5 to 8:30, I’ll be on my cell; for the rest of the weekend, call me at home.” And having to change phone numbers when you switched jobs or cities.

GrandCentral’s solution was to offer you a new, single, unified phone number, in an area code of your choice. Whenever somebody dialed your uni-number, all of your phones rang at once. No longer did people have to track you down by dialing multiple numbers; no matter where you were, your uni-number found you. And all voice mail messages landed in a single voice mail box, on the Web. (You could also dial in to hear them as usual.)

On the Web, you could play back your messages or even download them as audio files to preserve for posterity. You could even ask to be notified of new voice mail by e-mail. But wait, there was more. Each time you answered a call, while the caller was still hearing “one ringy-dingy, two ringy-dingies,” you heard a recording offering four ways to handle the call: “Press 1 to accept, 2 to send to voice mail, 3 to listen in on voice mail, or 4 to accept and record the call.” If you pressed 3, the call went directly to voice mail, but you could listen in. If you felt that the caller deserved your immediate attention, you could press * to pick up and join the call. This subtle feature saved time, conserved cellular minutes and, in certain cases, avoided a great deal of interpersonal conflict.

GrandCentral also let you record a different voice mail greeting for each person in your address book: “Hey, dollface, leave me a sweet nothing” for your love interest, “Hi, boss, I’m out making us both some money” for your employer. You could also specify which phones would ring when certain people called. (For the really annoying people in your life, you could even tell GrandCentral to answer with the classic, three-tone “The number you have dialed is no longer in service” message.) Also very cool: Any time during a call, you could press the * key to make all of your phones ring again, so that you could pick up on a different phone in midcall.

If you were heading out the door, you could switch a landline call to your cellphone.GrandCentral also offered telemarketing spam filters, off-hour call blocking (“never ring my BlackBerry on weekends”), and a dizzying number of other functions.

For people with complicated lives, GrandCentral was a breath of fresh air. It felt like a secret power that nobody else had. Then, in 2007, Google bought GrandCentral. It stopped accepting new members, ceased any visible work on it, and, apparently, forgot about it completely. The early adopters, several hundred thousand of them, were able to keep using GrandCentral’s features. But as time went on, their hearts sank. In January, Salon.com summed it up in an editorial called, “Will the Last One to Leave GrandCentral Please Turn Out the Lights?” As it turns out, the joke was on them.

Google was quietly working on GrandCentral all along. Starting Thursday, existing GrandCentral members can upgrade to Google Voice. In a few weeks, after debugging the system, Google will open the service to all.Google Voice starts with a clean, redesigned Web site that looks like an in-box, à la Gmail. It maintains all of those original GrandCentral features — but more important, introduces four game-changing new ones.

Free Voice Mail Transcriptions From now on, you don’t have to listen to your messages in order; you don’t have to listen to them at all. In seconds, these recordings are converted into typed text. They show up as e-mail messages or text messages on your cellphone. This is huge. It means that you can search, sort, save, forward, copy and paste voice mail messages. No human effort is involved; it’s all done with software. As a result, the transcriptions are rarely perfect. For one thing, Google’s software doesn’t seem to have discovered punctuation yet. (“ohh hi it’s michelle i just wanted to let you know that i really had fun last night and it’s really great to see you okay talk to you later bye bye.”)There are errors, of course; it’s hard enough for people to understand cellphone conversations, let alone computers. Cleverly enough, the Web site displays transcribed words more faintly (light gray) when it is less confident about the transcription. Fortunately, it generally nails numbers — phone numbers, arrival times, addresses. And the rest is accurate enough to convey the gist.