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T-Mobile devotees are in for a treat tonight, as the carrier has confirmed that it will be making its popular Wi-Fi calling feature on supported Android phones completely free — meaning it won’t continue to use up any cellular minutes.

But while it’s certainly great news for T-Mobile customers, free Wi-Fi calling is likely one of many features that will get dropped if AT&T’s $39 billion merger with the carrier goes through.

T-Mobile announced last October that it would bring the Wi-Fi calling feature to Android phones after making it available on select BlackBerry devices for some time. It’s been rumored for a while that T-Mobile would make Wi-Fi calling free, but today Kineto Wireless, the company that powers the feature, confirmed that it’s actually happening. T-Mobile swiftly acknowledged the news in a statement to GigaOm, saying that it will be offered to Even More and Even More Plus plan subscribers.

Kineto uses UMA technology to turn wireless hotspots into extensions of T-Mobile’s network. That allows T-Mobile’s phones to seamlessly jump to your home Wi-Fi network when you’re in range. It’s a smart way to extend cellular reception, since pretty much everyone has a Wi-Fi network at home these days. It’s also a more elegant solution than AT&T’s 3G Microcell (and other similar femtocell devices), which now costs $199 (up from $149!) and eats up cellular minutes.

But of course, you can bet AT&T will put a stop to this elegant feature if regulatory agencies approve its T-Mobile deal. AT&T has made it clear that it will consolidate its network with T-Mobile’s over time, which means Wi-Fi calling and other cool aspects of T-Mobile’s network — including its cheap $15 tethering and plans for ultra-fast HSPA+ speeds — will get killed off.

T-Mobile, the smallest of the four major US carriers, is offering these features as a way entice customers from its bigger competitors. But once it becomes part of the AT&T borg, there simply is no reason to keep them around. Hopefully, regulators are considering this as they determine if the deal is actually in the public interest.

For now, I suggest you take advantage of these features while they last. I’m even considering getting the lust-worthy Galaxy S II on T-Mobile myself — at least, whenever Samsung gets around to announcing that beauty on US shores.

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