Originally Posted by JK01!
Assuming you have a P9, does yours have the option to switch on vowifi under more?
I have b540 Oreo and it has somewhat different organization of Settings options.
Due to the hack to make it work with international 3G/4G operators, it does have VoLTE and VoWiFi settings, but it is known that VoLTE cannot really be enabled (guy who worked on the patches to adapt Chinese b540 for international users stopped developing them, and VoLTE was the last known thing that remained unsolved and there were not many people asking for VoLTE on the b540 Oreo - all other patches are known to be working).
If it 'helps' you, attached are VoLTE/VoWiFi settings from the patched Oreo b540, but that's all from me about the topic