LTE Phone can't be upgraded to VoLTE...

As the above heading will give u brief idea that LTE compatible phone can not be upgraded to VoLTE but what are the reasons..
Why my LTE supported phone can't be upgraded to VoLTE??
Why use a separate channel?

Technically speaking VoLTE is a SOFTWARE ONLY FEATURE. It runs on the IMS Backbone, which provides other services like SMS, ViLTE over IP infrastructure. So why do we see specifications of mobile quoting that VoLTE is supported in hardware, or we find answers that native VoLTE cannot work on existing 4G phones like iPhone 5s.

Although I am not an expert on this, I'll try to lay it down in as simple words as possible.

But before going VoLTE, I would like to bring to attention few terms that actually are the root cause of this VoLTE in hardware confusion. one is called: -

Quality of Service (QoS). In short, 4G standards require the services provided to give some sort of assurance of quality, say e.g. the link speed or bandwidth you get for a certain service shall not drop down a certain level, etc.

Why not the Internet?

Apart from the obvious security reasons of exposing a private intranet on public Internet, the other reason is the QoS. Yes! You cannot control the quality of a service you don't provide. So there is little to none Quality Assurance on the default APN as it serves Internet. So in order to provide the much required QoS for voice service, IMS and VoLTE are on a separate bearer.

Another term is Access Point Name (APN). Simply putting, these are addresses of various IP Gateways that provide IP/Tunnel connectivity for different services. These are also present in 3G and 2.5G networks and are so responsible for providing you with Internet/WAP and MMS access in GSM. But since 4G is an IP only network, APNs play an even greater role.

Now normally each APN provides access to a gateway that connects us to various different IP networks including but noṭ limited to Internet, IP-based Multimedia Services (IMS), corporate DCNs etc.

Now coming back to VoLTE, as I have already stated that VoLTE runs on IMS backbone, which is just another IP network behind a Gateway. So in order to reach that gateway, you would need an APN.

Now every 4G device has data connection by default since there is a default APN configured for all, the gateway for which is assigned by default as soon as you switch on the mobile. But normally there is no connection, no way to reach the private IP network on which IMS is running, since we cannot expose the same to an insecure public network like the Internet.

So how can we then reach the IMS? Via a secondary APN, that is available with the phone so that it can connect with the IMS gateway. These connections are called bearer. So for Internet, you need a bearer and for IMS you need another bearer.

Now this is where the problem arises, older LTE chipsets do not have the capability to establish multiple bearers. So the phone is stuck connecting with the Internet only which is the default bearer.

Modern chipsets have the ability to establish two bearers, default for Internet at device boot, and secondary for voice while attempting a call. The vendors just need to put the code to do so in their respective Radio Interface Libraries, so that it can communicate with the LTE chipset application processor to establish IMS bearer.

But IMS is all IP right?

Seems like VoLTE is hardware dependent after all! Then why did I say VoLTE is a software only feature? I still say that VoLTE is software only as it is only dependent on IP, but it is responsibility of the hardware underneath to provide the VoLTE stack that additional IP access to the IMS gateways along with default Internet access. It's the same as any other computer software that depends on presence of two NICs to access two different networks. Basically 4G internet and IMS are TWO DIFFERENT PACKET DATA NETWORKS!

VoLTE is hardware dependent one way or the other? Actually no! It's because an operator can decide to expose an IMS endpoint on the public internet and design an application to connect to that endpoint to give it the ability to call. But this call although is VoLTE call, is being routed over the default internet bearer, on which quality control is not maintained. So your voice experience is gonna be as good as your data experience since in the absence of QoS, you cannot ensure quality of the call. But at least you get something, and won't have to throw away your older 4G data only smartphones.

So I guess that all about it, VoLTE is software only feature, but is dependent on the hardware to provide it access to run. VoLTE can virtually run on any computing device as it's basically a beefed up version of VoIP (Skype) that we have been much fond of.

I hope the explanation helps.