If you haven't heard, the value of a minute of voice time on AT&T's network just tanked, at least for iPhone users, with last week's release of Viber, a new free VoIP app just released on the iTunes App Store. Viber allows iPhone users to place free calls nationwide (no strings) over 3G and Wi-Fi. It does this without requiring registration, so that when you call someone, they actually see your number, not one the service has assigned to you.

Viber offers seamless integration with contacts, delivers excellent voice quality and mimics the iPhone's native voice interface to perfection. This is the kind of ease of use and simplicity to which I'm guessing Google Voice and Skype aspired but in the end couldn't achieve.

While you could say that Viber represents just another VoIP app, I think it's more than that. The first thing I did after downloading it was to call my wife and tell her to download it on her iPhone 3GS. Then I called AT&T and downgraded our voice plan from 1,400 minutes per month to 550 minutes per month. That's a savings of over $20 a month, and I'm guessing I'm not the only one of AT&T's iPhone users who just realized they could save a buck or two by using Viber.

Since the release of Google Voice, I've been wondering how carriers are going to account for customers making voice calls over 3G. Voice is still a big deal for carriers and accounts for the majority of their profits, as the high costs of network maintenance and expansion keep data profit margins relatively low. So what does it mean if legions of smartphone users begin downloading apps like Viber, slashing not only their minutes but AT&T's revenue stream as well?

Viber says it has BlackBerry and Android versions of the app coming, so this isn't something that will be limited to AT&T, provided Verizon and the rest of the kids on the block allow the app on their networks. Lord knows "net neutrality" is still tongue-and-cheek with the carriers.

It'll be interesting to see how carriers market voice over the next couple of years. They have to be aware that the days of charging for a bucket of minutes with a straight face are limited. As it stands now, carriers charging for a minimum number of minutes is comparable to a fast-food joint requiring that you purchase a drink with your meal, even if you don't want one.