Google Voice allows you to point your number at any other number you like, so if someone calls your GV number, you can make that go to your cell, home, or whatever you want. The problem is, when you call from, say, your cell number, the number assigned by your carrier will be what shows up on caller ID on the phone you are calling. Enter the Google Voice App. This app will let you call from your cell phone and have your GV number appear on caller ID.
Of course, it is chock full of other features too, like SMS management and voice mail. It is almost like a phone service shell for your phone. You are still using your carrier though, so this isn't a VOIP app that is bypassing talk time minutes.
There are a number of very good reasons to have a GV number, besides the fact it is your number for life. I moved across country but wanted to keep the phone numbers I had from two moves ago. My carrier was willing to do that, but it presents a problem in limited circumstances. Many school districts have long distance calling blocked, so my kid's schools can't easily call me or my wife since our phone numbers have area codes from two thousand miles east of here. We got GV numbers with local area codes to prevent that from happening. That use sort of contradicts the number for life feature since I'd get the GV number changed if I moved again for similar reasons. It points out however how a service like this can be used in many different ways, not to replace your carrier, but to work in conjunction with it and make your life easier.
AT&T isn't having any of it though. It is their network and they will treat users how they wish. They have a lot of leverage too because if you want an iPhone, 99.9% of the people in the US will have to get it from AT&T. Jailbreaking isn't something that most people will fool with. AT&T isn't the only one though that treats their customers that way. All of the US carriers do. They are terrified of being reduced to a dumb pipe just like the ISPs did. They are scared of losing the branding they have built up.
I can appreciate that. But I wonder if they can appreciate the amount of animosity and resentment so many users have against their carriers for imposing these limitations on their users? There has to be a middle ground where power users can harness the power of all of these services without it taking money out of the carrier's pockets. Until that happens, when some people say the word "carrier" they will continue to get that look on their face, as if they had just said "used car salesman" or "ambulance chaser."