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I think that one of T-Mobile’s most under-appreciated features is WiFi calling: the ability to use your phone anywhere that there’s a WiFi signal, regardless of actual cell coverage. Not only does this make international roaming essentially free (as long as you only call while on WiFi), but you just get great coverage in general without ever having to worry about ordering a portable femtocell from your carrier. Furthermore, WiFi calling also supports text and MMS over WiFi. But how well does this feature work on the new HTC One S?

WiFi calling on T-Mobile has been around for a few years now, although this is the first time I’ve gotten to play around with it. When it first appeared on Android, there was a special app that enabled the feature, but on the One S it’s baked into the OS and is included in the Wireless & networks menu. There are also three modes you can select: Wi-Fi Preferred, Wi-Fi Only, and Cellular Preferred.

I found it interesting that when selecting Wi-Fi Preferred or Wi-Fi Only, the actual cellular radio would turn itself off. This is great, as it offers peace of mind that you are actually using WiFi to make your call (this is important if you are traveling internationally). I didn’t really notice a difference in performance between these two settings, as they both seemed to accomplish the same thing, but perhaps Wi-Fi Preferred is quicker to reconnect to a cell tower if your WiFi signal is low or unreliable.

When WiFi calling is turned on, you’ll notice a persistent icon in your notification tray, reminding you that you’ll probably still use your plan’s minutes, even on WiFi. When you dial a number, the call screen looks exactly the same as normal; the only difference is that the WiFi notification turns green.

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I couldn’t notice any difference in sound quality between regular calling and WiFi calling. There was no discernible lag, and if anything the call might have been slightly clearer over WiFi. Your miles will definitely vary, though, as you’ll definitely need a strong WiFi signal to keep the call quality up.

The only hiccup I experienced was when I first tried to set up WiFi calling. T-Mobile requires you to register a 911 address in order to use the service, although their website simply would not accept what I was entering. A quick call to customer service fixed this, but it was a hassle nonetheless.

I have also observed that the battery seems to drain faster when leaving WiFi calling on all day. The phone will make it through most of the day with little to no use, and only half a day if I’m browsing the web, playing games, or otherwise just keeping the screen on. If I turn WiFi calling off (but leave WiFi on), the battery lasts much longer (about a day and a half, with moderate use).

T-Mobile has previously stated that it is aware of the battery drain issue with WiFi calling, and an update is on its way. (In the meantime, it suggests simply leaving the phone plugged in while using WiFi calling.) That’s great news, because otherwise this feature works flawlessly, and really sets the T-Mobile One S apart from the rest of the HTC One lineup.