Are here any Volla Phone users? What are your experiences?

I had a Volla Phone. Never again.

There is indeed a dual boot VollaOS/UbTouch. However, even if the device is GSI compatible you cannot install any other Android ROM on it. But that's a personal choice you might not care.

Now my problem stems from hardware reliability/customer service.

My Volla Phone's acreen stopped working for no apparent reason. Touch would not be recognised and/or the touch screen would register many touches when there were none. My phone was under warranty, it had been bought less than 6 months beforehand.

I sent for it to be repaired. They held it for more than a month and a half before sending me an email, without any prior invoice that stated : "the phone won't be sent back to you unless you pay 200€ for the repairs".

So yeah, no more. I'd advise you to get another phone.