For example - here in Australia (but also world wide in most cases):
To be clear, most of our devices are indeed VoLTE compatible when it comes to hardware & bands etcā¦
However, most cellular providers have standards (config files) that need to be met to enable VoLTE to work.
LTE data is no problem.
There is also the fact that your watch may appear to be using 4G for calls - but in reality it is switching to 3G automatically for voice calls.
You can test this by selecting 4G only in the watch engineering mode settings (research if you are unfamiliar with this).
If you set to 4G /VoLTE only for sim card and you cannot make voice calls - this is why. Your sim provider is not compatible with these watches.
Please take time to understand what this might mean for you.
We are researching what settings (configuration changes) are needed to get VoLTE to work, but itās a big project and feedback is needed.
Cheers
Pablo11.
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Hello Pablo,
After a long lead time, sorting my daughterās samsung A13 on a TPG sim, I have been able to test her sim in my APPLLP 4 pro.
I am very pleased to say the sim status reported LTE voice and LTE data where my current telstra (prepaid) sim reported 4g voice and 4g data. (Initially the TPG sim reported HSPA+ but it only took a second or so to change to LTE for both).
We could exchange calls and text without issue. Her samsung reported 4g voice and IWLAN for data but had the VoLTE symbol on her main screen using my Telstra sim.
I hope I am correct in assuming this means the settings on a TPG (Vodaphone) sim allow VoLTE on my Lokmat APPLLP 4 pro Z40.
I have been doing a lot more surfing on the xda forums relating to getting pixel and various chinese phones to use VoLTE in various countries, including Australia (Telstra). Many are developing workarounds to toggle carrier checks, or to force LTE on their phones, then some have MBN (?) settings from other carriers that they put into their phones (developer mode?) that work where their normal sim doesnāt.
I will collect links to these discussions and post so that folk with more understanding of the way this behaves in a phone may be able to confirm of debunk their assistance in keeping our phones functional for voice calls after 3g closes.
If I have posted this in the wrong place, I am sorry but it was a glimmer of hope that there may be a relatively simple solution to keeping our watches as phones for the life of VoLTE. I have been so satisfied with having a phone on my wrist and not in my pocket, even if it now means I have to change carriers and pay the same each month for a service that I could happily renew once every 6 monthsā¦ maybe ( I am stalking an eBay auction for a Motorola G54 5g which will have to bulk out a pocket instead of living unobtrusively on my wrist.).
I have a series of generic dialer codes that once input report on functions that users canāt access if the os overlay doesnāt allow it. One is for the toggle between carrier check, but I wonāt post these as they may be device specific and I wouldnāt want anyone to damage their equipment. I havenāt tried them either.
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The best way to be absolutely sure about VoLTE working correctly is to go into the unisoc engineering mode and select LTE only for the Sim configuration.
Turn off Wi-Fi and then if voice calls work you can be sure that volte is working correctly.
The reason is that some providers allow access to their volte network without custom settings. Unfortunately though, almost all of them do not allow this.
Sadly, fixes for enabling no network checks for volte requires root access to edit the build.prop etcā¦
Have you checked to see if the āEnable VoLTEā switch is available in the watch Sim card settings when you are using the TPG Sim?
This toggle only appears if the Sim you are using supports no carrier config for LTE. It will not be visible using a telstra Sim, and it would be great to know if you can see it when you use TPG Sim.
I donāt know if you have already seen this article, but it helps explain a few things.
The issue with most of the fixes that are discussed on the Web is that they only work for Qualcom or other widely used mobile phone chips.
Finding a fix that doesnāt need root for Unisoc - SPRD chips is going to be extremely difficult 
https://xdaforums.com/t/guide-enable-volte-icon-in-mobile-network-setting.3764826/
1 Like
Thanks for the article I will look through it to better understand what is at play.
I hope the TPG sim shows the no carrier config. I also hope I am looking at the correct settings - sim status? Or is it network config?
Iāll check both and see.
I have just swapped the TPG sim into the APPLLP 4 pro.
Saddly, while the sim status reports LTE for both voice and data on the TPG, (where the Telstra reports 4G), in the network settings neither has any reference to select āEnable VoLTEā.
Some what strange was the TPG and Telstra sims only offer options to select: 2G ; 3G 2G auto ; 4G 3G 2G auto , but the Telstra sim has a toggle for 4G calling to " Use 4G services to improve voice and other communications (recommended)", where the TPG sim didnāt.
My understanding was that TPG uses the Vodaphone network, which closed 3G quite some time ago. However, ensuring emergency calls in a rural area may have clouded this. Strange that the watch would have that toggle for Telstra only, when Telstra was notifying me that my watch would lose voice calling when 3G closes, and nothing for TPG.
I will have to build up a lot more courage to go into the enginering mode to force LTE as the last time I tried I lost alot of other functions while trying to sort out the unwanted notifications from other peoplesā poorly set up apple devices.
I will see if anyone is willing to allow me to trial their Optus sim in the mean time.
I had a go at opening the unisoc engineering mode to do the confirmation test, but nothing in the telephony menu looks like it selects LTE only. There are a lot labled VoLTE but they appear to be for inputting and applying settings.
I used the code: * # * #83781# * # * for spreadtrum / unisoc / ZTE. (No spaces though. Something in the posting pane didnāt recognise the * symbol unless it was spaced.).
Sorry if I have simply missed the menu. Nothing looks close to the toggle lists I have seen in my internet searches.
Some new infoā¦
Some more information from Telstra regarding their knowledge of my watch. This was their updated email detailing the reason to buy a new phone.
They now identify the appllp 4 pro as:
A 4G device with limited band support. 4G non VoLTE device.
Telstra suggests that best compatability will be achieved if the device uses: LTE 700Mhz / B28 capability.
Time to go back to the specification sheet, and venture into engineering mode again I suppose.
I got a Z40 smartwatch and itās missing only 3 T-mobile bands which is LTE-FDD Band 66 and 4 along with WCDMA band 4 too. My internet data works perfectly on the watch but calling is very unstable. Is there any way to fix this. In case itās not possible thereās one solution I still have though and that is that 2g calling works perfectly so if I can use a automated app for switching to 4g only while I use the internet, it can give me a stable call. However I doubt this solution would work for long of course as T-mobile will soon remove 2g from California.
This is the first time Iām glad I live in a third world country lol, weāre JUST getting 5G in our biggest cities so we are still far apart from a 2G/3G shutdown. Movistar just recently enabled VoLTE in my city, but I havenāt tested it on my Optimus 2 yet.
Sorry, no way to add extra bands to the modem chip.
Set up in factory as per OEM request.