We’ve seen Wi-Fi calling in the past from carriers and it usually involves having a separate router-like device that you need to “rent” from your operator. The problem with that system, besides the extra cost, is it’s not mobile so it only works at home (as opposed to a friend’s house or your local Starbucks).

T-Mobile has a new Wi-Fi calling initiative and the low-cost Nokia Lumia 521 is the first Windows Phone to have the feature (the T-Mobile Lumia 925 will have it too at launch, and perhaps the Lumia 810 may get it with its Amber update later this summer).

So how is it? It’s actually one of the coolest things we’ve used that has improved mobile connectivity. Sure, it making calls this way still counts towards your minutes, but it goes a long way in improving reception in certain cellular dead-zones.

By allowing you to make/receive phone calls and send text messages all over your existing data connection, T-Mobile has really upped the ante for competition. Since the service is on-board the phone, it also means wherever you have a Wi-Fi connection, you can make a call—home, your parent’s place, a hotel, Starbucks—it doesn’t matter.

Call quality is fantastic and the connection has no discernable delay. Over the weekend, we used the phone at a location with literally zero bars on T-Mobile resulting in a “no signal” status on the Phone tile. By utilizing the Wi-Fi calling feature, we all of a sudden had a fully functional cell phone again.

Users have three options, including Wi-Fi Preferred (makes calls over Wi-Fi, falls back to cellular), Cellular Preferred (makes calls over cellular, falls back to Wi-Fi) and the most interesting: Wi-Fi only. That last option means the device will only connect via Wi-Fi for phone calls and never via cellular. 

http://www.windowscentral.com/psa-update-your-nokia-lumia-521-today-get-wi-fi-calling

We’re very excited about using this feature on our Lumia 521 and we’re even more thrilled with it coming to T-Mobile’s Lumia 925 later this summer. It’s a game changer for us in how we use our phone as it costs $0 to use or add to your plan (though you may need T-Mobile to configure your SIM).

Unfortunately, this feature is 100% dependent on carriers, meaning no, this won’t roll out to other Lumias and it is not a Nokia feature per se. In other words, free Wi-Fi calling may be a new area for carrier competition and something to consider for your next purchase.

You can read more about T-Mobile’s Wi-Fi calling here and make sure you watch our demo video of the service in action above. To enable Wi-Fi calling, users must have the latest firmware on their Lumia 521 and must do a hard-reset to install the necessary software.