it's a value added service/technology to cover all the blind spots which mobile network may not be strong/stable....

Despite on wifi, you are still connected to Maxis network and they are still offering you the voice services. When your friend calls you, you still can receive the call …

Mobile technology usually rather poor indoor, and voice will also be poor, vowifi can give better call experience.

There is option to disable vowifi if one insist to have worse call experience but you have to ask yourself whether it is eventually what you prefer?

So instead of dwelling into whether Volte or Vowifi, the real question is which one delivers better call experience… for my case one 1 bar LTE at home, Vowifi improves my call experience significantly…