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However i.e. have seen occasisional instances that unsupported phones have worked.
I assume you've checked with Vodafone customer service to see if the feature is toggled On in your account and you've had a look in your myvodafone ?
Also that you have the needed contract type to have Calling Over Wifi.
If you take a look near to the end of the link page there is advise on how to set up the feature.
In regards to voLTE then you need to be in a supported area on an accepted contract type and using a supported model of phone.
"Apple devices can be used from any retailer, however all other devices must have been purchased directly from us. "
May i.e. assume Vodafone didn't directly supply you this model of phone ?
Full Info is in this link Who-can-get-4G-Calling.
You own a very nice feature rich packed phone but unfortunately it's not a main stream model that's in the bracket of being supported as far as i.e. can see.
I wish you all the best with this situation.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung Gear s3 Frontier Watch.