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At WWDC Apple unleashed FaceTime on 3G networks. While you would think a high-quality video chat and collaboration service would be poison to consumers’ restricted data plans, there’s another way to look at it: Compared to what mobile operators charge for voice, FaceTime(s aapl) is really a bargain.

When FaceTime first emerged, 9to5Mac performed some 3G tests on jailbroken iPhones to determine just how much bandwidth the app would consume. It’s findings: 3 MB per minute. So if you had a $30/2 GB plan from Verizon(s vz)(s vod) you could feasibly support 666 minutes of FaceTime calling, while a 3 GB plan at the same price from AT&T(s t) would give you an even 1000 minutes of video talk time.

Now look what those same carriers charge for voice minutes: For base level voice plans both AT&T and Verizon charge you me $40 for 450 minutes and $60 for 900 minutes. Making a FaceTime video call winds up being a heck of lot cheaper than a straight up phone call.

Before you get the bright idea of replacing your voice service entirely with FaceTime and a data SIM card, know there are some limitations, the biggest of which is FaceTime only works between Apple products. 3G network coverage, even over Verizon’s extensive CDMA network, can be spotty, and just because there is a 3G signal available to you doesn’t guarantee you’ll get enough of bandwidth to support a video call. Carriers’ circuit-switched voice services may be expensive, but for the most part they work — a claim we can’t yet make for 3G.

Of course, supplementing your voice service with FaceTime might not be a bad idea especially if a lot of your contacts own iPhones, iPads and Macs. Knocking $20 to $30 off your minute plans (AT&T and Verizon both sell unlimited voice for $70 a month) and replacing it with the next higher tier of data could wind up saving you a lot of money. Many consumers might not even need to bother with a data plan upgrade. According to a recent report from Ericsson(s eric), iPhone users only consume on average 350 MB of month of data over global HSPA networks, which puts them well out of reach of most data caps.

As for the operators — who already claim their 3G networks are overtaxed — this is probably not an ideal development. Not only would a sudden deluge of FaceTime calls load up their networks even more, the service could start impacting their voice revenues. Since they make far more for on a minute of voice than they would on a minute of FaceTime data, they have far more to lose.