iPhone 6 Plus falling back to 3G for voice calls
Cellular communications have traditionally used separate voice and data channels. But with the advent of smartphones, bulk of the communications now happen on the data channel. It's no coincidence that only the data channel remained by the 4th generation (4G) technology. To carry voice, though, a fallback to 3G or older technology's voice channel was initially needed. But VoLTE (Voice over LTE) came onto the scene to do away with this by letting the data channel handle voice as well.
As with many technologies, Korea was a forerunner in adopting VoLTE. World's first commercial VoLTE service was launched more than two years ago by two of the three major Korean carriers, SK Telecom and LG U+. The remaining one, KT, joined mere two months later. Worldwide adoption didn't really take off until earlier this year, however. About a dozen carriers around the globe now have it. Noting this trend, Apple has decided to enable the capability in the iPhone 6 series.
So naturally, I wanted to see VoLTE on my iPhone 6 Plus in action. It can be enabled in the Settings app by going to Cellular and selecting "Voice and Data" option for LTE. The previous 5/5C/5S models simply have toggle switch for LTE instead, even with iOS 8.0 update. After that, I made voice calls to a lot of different lines, including a VoLTE-enabled phone on the same carrier. But every time, the connection indicator turned from LTE to 3G, showing that VoLTE wasn't working.
iPhone 6 Plus seen as "OMD Default Handset"
I checked to see how the network recognized my iPhone 6 Plus. It was being listed as an OMD Default Handset, the same name that my iPhone 5S was given when it was brought in from the US. In other words, a generic device that it doesn't recognize. Perhaps the network couldn't tell whether the device had a VoLTE capability, so it simply ignored its attempts. It was time to visit my carrier's branch office to get to the bottom of this.
Visiting Gangnam branch of SK Telecom again
Upon arriving at the Gangnam branch of SK Telecom(SKT), I sat down with a customer support staff and told her about the situation. According to her, the VoLTE Device Portability policy had been in affect since July 1, 2014, which enables VoLTE phones from other carriers that came out after that date to have VoLTE working on SKT. So she was baffled as to why it wasn't working in this case. I suggested that maybe the device had to be recognized by the network first.
iPhone 6 Plus seen as "OMD Apple VoLTE"
Thus she proceeded to register my iPhone 6 Plus on the SKT network. She found that a device code named OMD Apple VoLTE had been added on September 23, just a day before my visit, and she assigned this to my phone. I did a quick airplane mode on/off cycle just in case, and made a call.
iPhone 6 Plus makes a VoLTE voice call
And what do you know, it worked! The connection indicator stayed at LTE the entire time I made a call. VoLTE at my end worked regardless of whether the other end was VoLTE or not, although I could hear a clear difference in quality when the receiver also had a VoLTE-enabled phone. The sound was crisp, like when I used an mVoIP app like Viber.
By the way, there's a strange exception to an otherwise full VoLTE experience. If I make a call to an internet (VoIP) phone (number starts with 070), the indicator goes to 3G like the old times. Both ends are IP-based, yet it's the only time it has to go through a middleman. I'll have a look into this when I have the chance.
Meanwhile, I have confirmed that the situation with those using KT instead of SKT is largely the same. Unregistered iPhone 6 series will have LTE enabled when a compatible SIM card is inserted. To use VoLTE, you need to have the device registered to the network. In the case of LG U+, the phone will theoretically work on its network but it is blocked until the official launch in Korea.