Earlier today, T-Mobile announced that it has pushed the button to launch its Voice over LTE service in Seattle. This means that Seattle-ites will be able to make regular voice calls using an LTE connection, and not have their phones switch to an older network technology to do so.

One slight caveat: Only specific phones will be able to make use of the service right away. T-Mobile stated that – after a software update – the Galaxy Note 3, LG G Flex and Galaxy Light would all be able to take advantage of VoLTE.

One question we’ve been asked after the announcement – and has come up elsewhere online today – is whether or not the iPhone will support this new service. And the answer is: Probably not.

But, the iPhone is technically capable. It just comes with a compromise. One of our regular followers, Joe, got in touch with us. Turns out the iPhone 5, 5s and 5c has some coding in it pointing towards VoLTE. The code is within the baseband itself, or the firmware of the actual chip Apple uses for LTE connections. Specifically, the Qualcomm MDM9615m, which is used in all of Apple’s LTE devices, including iPads.

As you can see, VoLTE is specifically mentioned within the iPhone chipset’s own firmware. So, the chip itself is capable of making calls over LTE. The reason it doesn’t do so right now is because Apple/T-Mobile haven’t released a carrier update to make it happen. And the sad news: It probably won’t.

A more in-depth look at T-Mobile’s carrier settings (above) reveals a string of code related to the way LTE can behave. This particular code string is found within the overrides PRI file. This file exists for every carrier, and tells the phone how to behave on the carrier in use.

For current LTE devices there are two kinds of calling methods. The one most often used, is the one displayed here. It uses VoCSFB, referred to here as “CS” or “Circuit Switched”.

Currently “CS Voice Only” would indicate that it’s forced to use the Circuit Switched domain. This tells your phone to use older network technologies for phone calls when you have an active LTE data connection. The rule ensures that if you receive a phone call while you’re using LTE to browse the web, or use data in any fashion, it’ll downgrade to 3G/HSPA+, to ensure that you can keep both connections running at once.

Bliss.

The carrier bundle could be altered to change this rule, so that voice calls could be made over an LTE connection, taking advantage of the chipset’s VoLTE capability. #winning

Here’s where I get conflicting arguments chatting with our various network and bandwidth gurus. One believes a carrier update is all that’s required to get this service running on an iPhone. VoLTE and any other data connection should work just fine at the same time. In the same way that you can take a FaceTime audio call over LTE, or have background data or iMessage while browsing the web, you should be able to receive a VoLTE call browsing the web.

“Internet data is packet switched domain as well. Wireless networks have two subsystems: a circuit-switched and packet-switched subsystem. The circuit-switched subsystem (called a domain by some) handles SMS and voice calls. The packet switched subsystem/domain handles the multi-media aspect of MMS (the message itself and signaling is handled by circuit-switched domain), and also handles IP data sessions (which is what VoLTE and internet data run on). LTE is special in that it lacks a circuit-switched domain to handle voice calls and SMS.”

Our second school of thought is that the LTE chip in the iPhone isn’t capable of running voice and data connections simultaneously. I say that, technically, VoLTE is a data connection, so in essence the iPhone can’t have a VoLTE call going and have you browsing/streaming at the same time.

Addressing the issue of the difference between FaceTime and VoLTE:

“Because FaceTime is a service that isn’t hosted by the carrier and doesn’t occur in the carrier’s IMS Packet Switched domain, which only allows for carrier specific packet data like texts and picture messages. FaceTime simply uses pure internet data. It’s no different than having multiple web pages loading at once in the Safari web browser on the device. VoLTE is a carrier specific protocol hosted by the carrier and does occur in the IMS Packet Switch domain where texts and picture messages are also sent through.’

So if a carrier update was released to take advantage of the iPhone’s VoLTE compatibility, users would have to sacrifice simultaneous voice and data. You’d be browsing or streaming in glorious full-speed 4G, then receive a call, and the connection to your on-screen content would close to make way for the phone call.

Not bliss.

[Edit: It seems the general consensus – among those who know about these things – is that the iPhone should have no problem carrying a VoLTE call, and browsing/surfing/receiving other data at the same time.] 

Which ever of the two scenarios is correct in regards to simultaneous VoLTE and browsing/data, it doesn’t change the fact that VoLTE calls are technically possible.

There’s a case to argue that Apple could allow a carrier bundle to come equipped with a toggle switch within the Settings app to switch simultaneous voice and data on or off (if it’s necessary). All this would do is tell the phone to either use VoLTE for voice calls, or switch to the previously mentioned “CS Voice Only” command in the code and route any calls to the 3G network.

And then of course there’s battery life to consider. VoLTE is a more power hungry connection that the current CS (switch to 3G). And we all know the iPhone’s battery isn’t up there with the greatest.

Thanks, Joe/Neal.

[Updated: Added the edit above, and removed a couple of sentences from the end. I’m more than happy to admit my technical knowledge on these matters isn’t anywhere near the level of those who I’ve been talking to. I wanted to present an article purely to let concerned iPhone users know that their phones are capable of taking advantage of the new VoLTE service, also providing some insight in to any stumbling blocks.]