With pretty much every single phone on the market - Vodafone or whatever mobile phone operator you choose - will have their own customised version of the phone's firmware, only phones with their version of the firmware will have the correct settings to access WiFi calling on their network. Only phones they sell will have their firmware.
If you buy from elsewhere you'll have the generic manufacturers version of the firmware or even another mobile phone operator's version - so WiFi calling and VoLTE won't work. There is no technical reason why these features couldn't work - even though the networks will claim that it's a compatibility issue, it's not, it would be very simple to have it working. They just want you to be forced to buy your phone on contract, from them.
However the iPhone and the Pixel phones don't have this - they have one single version of their firmware that is controlled centrally by Apple/Google. The operator submits their settings and they're included in the software or downloaded on demand when a SIM from X operator is detected.
Therefore, the mobile phone company cannot control it anymore. Either every single Pixel 3 will have WiFi calling and VoLTE or none will have it. Every other operator has said the Pixel 3 will have WiFi calling and VoLTE. It's highly unlikely that Vodafone will say no and be the odd one out, especially if the Pixel 3 turns out to be a big seller.
The only way they could control it is to maintain a list of IMEI numbers (like serial numbers) and block it phone by phone like that - since they haven't blocked iPhones in that way, that would be a blatant prejudice and they'd look very bad - maybe Google would even intervene?
I'd say it's almost certain we'll get WiFi calling and VoLTE on our Pixel 3s - the only question is how long it will take them to get their a*ses into gear and do it.
