Really good job! I like the idea behind the effect and I think the method is very simple but effective. The only advice I could give you would be to use emphasise the coin vanish and give the spectator complete conviction the coin has indeed vanished. You could use a couple of hand washing techniques to achieve such a thing. Plus, I think by having the spectator place their hand on top of the phone, it can open up the possibility of toying with the phone before the climax. What I would do is ask one spectator to fulfill the role of watching my hands and asking another spectator to fulfill the role of watching the phone, similar to what Eric Jones does in his Sponge Ball routine. When it comes to the end, I ask the spectator "Did you see me touch that phone?" in which they will reply "No". I come back and say "That must mean I'm getting better at this!" and then reveal the coin under the phone.
Just my two cents, hope that's useful.
I can't see it now but I'll check it out after work 