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Excellent news!
I unlocked my Vodafone 6s Plus a few weeks' back.

Tried a Three PAYG SIM in it and VoLTE was alive & kicking despite you being told otherwise.
Excellent news! It appears that despite the best efforts of their shop staff (only works for three to three calls) and online chat support (only works on phones bought from Three) all iPhone users on Three get the benefit without any expended effort. #WIN :D
 
Note that it will only go onto 4g 800 VoLTE as last resort ie no 3g or higher freq 4g

Hi,

I know this is a really old thread but I wanted to make the comment that it is still relevant in 2020. My phone will only show VoLTE in an area where there's no Band 3 or 3G. This is still very frustrating and you'd think that Three would have got their Supervoice across all their bands four years on, but obviously they haven't!
 
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