Approaching the smoker, the waiter asked politely in a soft and friendly voice:
"Excuse me, Sir, didn't you happen to notice the sign displayed just in front of you?"
Exhaling a perfect ring of smoke, the jolly fellow quipped:
..." I did, but M'P MDM DPDBMMD AHDPP."
Rem: the puzzle's encryption was not by a simple substitution code , but rather via N(CODE)=N(TEXT)mod K + L or similar (like F(N)instead of N), where N is the ordinal value of a letter in English ABC and K, L integers to be found.