Move over, Republic Wireless: there's a new super-cheap cell phone carrier in town, although it has a huge catch. UppWireless, launching today in the U.S., offers 1GB of data plus unlimited calls and texts for only $15/month - but all of those calls and texts have to go through voice-over-IP apps.

In other words, it's a data-only plan for phones. You can also get 3GB for $35, or 5GB for $55. If you do your calling and texting through UppWireless's UppTalk app, it doesn't count against your data limit, but I can't see why you can't just use whatever app you want, including Skype.

UppTalk has a few advantages, CEO Jochen Doppelhammer said. For instance, on Android the app can hijack the dialer and texting apps, so that calls and texts go through UppTalk seamlessly. On iOS, the standard dialer just becomes useless; you have to dial through a separate UppTalk icon. (There's no Windows Phone version yet, but there's one coming soon.)

The VoIP app works well even over an EDGE connection, Doppelhammer said.

"Our VoIP is very bandwidth efficient, doesn't need a lot of bandwidth, [and] even on 64kbs it works fine," he said via email.

The texting app doesn't support picture messages to people who aren't using the same app, though.

The best deal here seems to be bringing your own T-Mobile-compatible phone, but UppWireless is also selling four phones: the Samsung Galaxy S III ($429), the Sony Xperia L ($239), the LG Optimus L9 ($215) and the Samsung Exhibit 4G ($169).

The ability to use any unlocked smartphone makes Upp more flexible than Republic, which only lets you use a few devices with special firmware. But Upp is the first U.S. service I've seen with no traditional, circuit-switched fallback for voice calling at all.

Much of Upp's success will be predicated on how well its VoIP app works, so I loaded it up on my own T-Mobile Moto X and tried to set it up. The outcome wasn't encouraging - I couldn't make it through the setup process. Doppelhammer said that while I was trying to set it up, the system was enduring a denial-of-service attack, and the damage should be fully repaired by the time you read this.

Hopefully, the app will work better in the future, but I strongly suggest you try to set it up on your existing service before considering UppWireless as a carrier.

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