Um, iPhones, Androids and Windows Phones all have at least one front-facing button, either physical or virtual.
You put a Z10 in a user's hands without the tutorial enabled and they're just helpless.
Like I said, if your hands are wet, or you sneeze on the screen, or it is raining, or whatever, virtual buttons fail.
I do wish smartphones phones came with a fallback "answer call" feature that didn't rely on the touchscreen being able to recognize your finger. I can answer a call on a flip phone with just my teeth.
I'll be skipping this Jolla iteration and I am sad b/c a 2nd is looking doubtful. When my N900 dies I'll be going retro w/ my flip-phone, until an open device with physical keyboard, larger screen and isn't designed to spy on me 24/7 comes around.