Skype and every other VoIP provider out there, and if these guys can reach critical mass, avid iPhone users will certainly be able to buy a mobile plan with fewer minutes. It works as such: install the free app on your iPhone, provide just your phone number (no "registration" is necessary) and then fire it up. Viber uses your existing contact and favorites list, and you can make Viber or standard voice calls from within the app; naturally, a Viber logo pops up beside any contact who is also a user, making it easy to see who you can VoIP call for free. As our brethren over at TUAW point out, there's simply no catch to be found -- well, except for those of you counting your kilobytes on AT&T's newer, metered data plans. Call quality was said to be excellent over 3G and WiFi, and potentially best of all, gratis SMS and an Android version are both on the way. Head on past the break if you need any additional convincing, and tap that iTunes link to get your download on. So, AT&T, what's the value proposition on those "rollover minutes" again?