You can flash SFOS on Volla with UBports installer. See here:
Scroll down and read the chapter ’ How to install Ubuntu Touch OS’
You have to download a flashing software (UBports Installer) to flash the Volla phone. The UBports Installer is available for Linux, Windows and Mac.
With the UBports Installer you have also an option to install SFOS. At the beginning the UBports Installer asks you what OS you want to flash onto the device, choose SFOS here.