I have always thought it was very plausibly done by a thick piece of glass.
As a kid, I jumped in a lake from a dock & the side of my leg by my knee hit the bottom. I ended up with a very wide, very deep gash with very clean edges. After fishing out the offending item with a net, I had apparently hit the edge of a broken beer bottle (thick glass). I had a noticeable scar for over 20 years. A fingertip meeting a similar piece of glass with high force/velocity? I can definitely see it. So, even without expert testimony or photos or xrays, it seems legit. Imo.
(And I think if he were banging the wallphone like suggested, it would have caused different, more widespread injury to his hand. Potentially a ragged tear &/or bruising or breaking of bones in the top of the hand. Again, imo.)
I am not a medical expert & am basing my opinions on my own life experiences.