Carriers are always advancing their network technology, but data usage has skyrocketed in recent years. That has made some segments of the wireless spectrum much more valuable than others to deploy new capacity and technology. Carriers are also exercising more control over their most forward-looking technologies, and that has the potential to seriously impact your phone choices. Let's take a look at how unlocked phones are faring in the LTE world.
VoLTE: Can you hear me now?
In the era of 2G and 3G voice, all you had to do was get a compatible device up and running on a carrier's network, and you'd be good to go. VoLTE is a bit different. This is a big step for voice calls on mobile devices (yes, people still do that) that moves to an all IP-based solution. VoLTE (that's voice over LTE) offers several times the audio bandwidth for vastly improved call quality. The eventual goal of VoLTE is to allow calls to be routed over the same radio as data. This is more efficient for the network and for the device.
That's great, but more often than not you won't be able to enjoy this feature just yet. All the big US carriers are actively deploying VoLTE on devices now. However, they mainly work within a single carrier network. So, if you've got a Verizon phone with VoLTE enabled, you can have a VoLTE call with another Verizon VoLTE user. Call someone on T-Mobile, and it will fall back to non-VoLTE. Carriers are still working on interoperability because implementing VoLTE requires that every "endpoint" (i.e. a phone) knows how to talk to all the others.
VoLTE is a loose standard that is designed to be flexible and powerful. That's why most carriers are doing more than simply routing voice calls. For example, many VoLTE systems integrate video calling and advanced text messaging. This requires the creation of new proprietary IP Multimedia Subsystems (IMS) by the carrier, and that increases the complexity of device certification and interoperability.
This all means that carriers have an even greater role to play in the implementation of software on the devices running on their network. In some ways, this is the way they like it. You might not, though.
Case in point: T-Mobile and band 12
The move to VoLTE on T-Mobile has revealed some very interesting aspects of how LTE voice services work. T-Mobile has been aggressively expanding its data coverage in the last few years after being far behind for a very long time. They're doing this with the aid of band 12 LTE, which the carrier calls "extended range LTE." This is in the 700MHz range, just like AT&T and Verizon's prime LTE bands. T-Mobile's "main" LTE band is 4 (1700/2100MHz), but band 12 is fast becoming king.
Band 12 is highly desirable for coverage because it penetrates buildings more readily than band 4, and in T-Mobile's case, it has allowed the carrier to expand coverage into more areas. T-Mobile has implemented band 12 for data, but also for VoLTE. There are areas where that's the only coverage T-Mobile has -- not even a 2G fallback. Therefore, to place a call in that area you would need VoLTE. If you phone has band 12 active, but lacks T-Mobile's VoLTE implementation (which requires certification) you could be in a situation where you have full bars of LTE, but all calls fail because the phone doesn't know to switch to roaming.
T-Mobile figured out last year that this could be an issue that got them in hot water with the FCC. If you have a phone that can't make an E911 call when it has service, the carrier could end up with a big fine. That's when the carrier started putting pressure on device makers to solve the problem. T-Mobile's solution: to make them turn off band 12.
In the last year, a number of high-profile premium and budget phones have launched without band 12 at T-Mobile's behest including the Nexus 5X/6P, Moto X Pure, Honor 5X, and 3rd gen Moto G. A few devices like the Alcatel Idol 3 and Moto E received updates after release that killed band 12 support. Some of these devices did get band 12 support in a later update, at least. Notice anything about all those phones? Yep, they're unlocked.
Because VoLTE requires certification, these phones also don't get LTE data on band 12 until they get certified. All the phones that T-Mobile sells directly are certified for all network technologies before release, but that's not the case when a company can just start selling an unlocked phone with T-Mobile's bands. This also slows down updates because T-Mobile has to look things over and make sure it plays nicely with their systems.
Some companies aren't playing ball with T-Mobile, like OnePlus for example. You can still get a OnePlus 2 with band 12 data and no VoLTE certification. Any company that wants to work with T-Mobile in the future is abiding by the carrier's wishes, though.
The future of unlocked LTE
We're approaching a time when unlocked phones could fall much more under the influence of carriers. T-Mobile is just the first to dig in its heels because of the way it has deployed band 12. When VoLTE is needed for voice calls, small OEMs might find their business model threatened. It takes time and money to get a phone certified on carrier networks, and that might mean higher prices for you. That's why cheap phones like the Moto G and Honor 5X aren't getting band 12 -- it's not worth the expense.
So why not make it easier to get VoLTE working without all the extra services and proprietary components? Frankly, that's not what the carriers want. Like ISPs, the carriers want to be more than a dumb pipe you connect to with your unlocked phone. They want to be the one supplying you with the phone, maybe on a lucrative early upgrade payment plan. Building these systems gives OEMs a reason to continue working with the carriers.
Unlocked phones won't go away as advanced network technologies continue to expand, but some of the benefits of an unlocked device might be lessened. They could end up more expensive to compensate for the cost of network certification, and your updates won't be quite as speedy.