@bolly : Thanks for the tip! I understand you convert the files with ffmpeg. Currently, I just transfer the concerned videos to my PC where VLC plays everything.
By the way, I'm still happy with my new Volla Phone.
But, it seemed to me the hardware itself should be able to play the H.265, that is what the chipmaker says at least.
I will probably ask Volla, or file a UT bug report.