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I asked the question a while ago in regards to iPhones and Calling Over Wifi when it first launched and compatibility.
I was using an iPhone 6 Plus then which can support that feature.
The answer was is that Vodafone are concentrating on supporting the newer models of iPhones in order to get the service right.
So right now the compatibility list is iPhone 7, iPhone 7 Plus, iPhone 6s, iPhone 6s Plus or iPhone SE which I'm sure your aware of.
I'm afraid Vodafone in my understanding will not be opening this relatively new feature up anytime soon or at all to iPhones prior to the ones I mentioned in bold.
I wonder how long it's going to be before Vodafone connect the number of customers they lose to issues like this.