7500 - I’ve been hi-jacked!
7600 - Radio out
7700 - Emergency / Mayday
And in the US, 1200 is the code for VFR flight. It’s 7000 in Europe, right?
And by the way, I was taught to change the numbers from right to left so that you never accidentally hit 7500 or 7700 even briefly. I sets off all kinds of alarms on ATC radars!!!