[Huawei Mate 10  Questions & Answers] Thasscas44:
Just got a Dual SIM Mate 10 Pro, and I have VoLTE working on Three UK.

I did a fair bit of research in to VoLTE / Wifi Calling (lots of lurking on these forms) before I got it, as access to SuperVoice would have been a dealbreaker if I couldn't get it (and other networks' full 4g spectrum).

It's worth understanding how Three currently structure their spectrum in the UK. I did a lot of head scratching when I used to have an iPhone, as despite Three texting me to say I was in a SuperVoice 4g area, my calls mostly dropped down to 3g.

Three have lots of high frequency 3g and 4g spectrum. They also have some low frequency 4g spectrum (band 20, 800Mhz), but there's a bit less of that.

Three's SuperVoice (VoLTE) only runs on the low frequency 4g 800mhz. The high frequency 4g does not carry VoLTE.

So - if you're on the high frequency 4g band (which is set by Three to be the first choice for locking on to) and you make a call, you need to get bumped to another band. If you're in range of 3g, and get so much as a whiff of it, it'll put you on that. If you can't get any 3g, but you can get lower frequency 4g (it travels further / deeper), it'll allow you on to that band and then - and only then - will you be able to make a VoLTE call.

Maybe one day Three will open up their higher frequency 4g to VoLTE if they restructure their spectrum, but I speculate that for the moment with the spectrum structured as it is, there isn't the justification for taking up high frequency 4g bandwidth with VoLTE call traffic, if there's increasingly unused 3g capacity sat alongside it. If however a phone is out of range of the high frequency 3g signal, the phone can use the low frequency 4g VoLTE for calling and, as Three put it, 'banish the indoor blackspots'.

My Mate 10 Pro is working in exactly the same way as my old iPhone used to - in that if there's any 3g whatsoever, it uses that for calls, often dropping to 3g from the higher frequency 4g (that doesn't allow VoLTE). It's only if you're somewhere where there's no 3g but there is 800mhz 4g, that you can use VoLTE. That makes testing it really hard! I've only had the Mate 10 for 2 days, and managed 2 VoLTE calls in a rural location. Near home I don't see VoLTE at all as there's very decent high frequency 3g coverage.

I also get the Wifi calling option in settings, however I haven't tested that yet, as I haven't been anywhere in the 2 days of phone ownership where mobile reception has been poor and there's been accessible Wifi to fall back on. There is no VoLTE toggle in settings, nor an icon in the status bar - the phone just quietly made a call over 4g (checked by using the 4g icon next to the signal bars, and using the settings>system>about phone>status>network menu whilst on a call to see whether the mobile network type remained 4g).

I'm hoping to test VoLTE on other networks shortly using my 2nd sim slot, however as the networks only allow VoLTE on contract it's not just a case of trying it out with a free PAYG SIM, unfortunately.

In terms of my phone, I ordered a Mate 10 Pro Dual sim Asian/International version (C636), that didn't seem to work with VoLTE (although as above it's hard to test with Three). If I'd have known that I could have bought a single SIM UK version and flashed new firmware to get dual sim I would have done to get a warranty, but when buying it I incorrectly thought maybe the hardware was different and I need 2 sims... I rebranded it to the Euro version (C432) and flashed Euro firmware, and I've since managed two VoLTE calls on Three UK. I'm hoping that other networks work as well... In hope of it doing so is partly why I bought the phone, and there are lots of reviews (that smell a bit of being paid for) that state Huawei has worked with networks to pre-install the VoLTE / Wifi Calling settings in to the Euro firmware.

Any questions just ask :)