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.