Pavlov-Classical Conditioning.
When an animal is expecting something, such as a dog with food...If music is played everytime the dog is eating, the dog will learn that anytime music is being played, it is time to eat. After months of being exposed to music while eating, the dog will salivate to the mere sound of the music, even if no food is present.....given your frog is not a dog...BUT, any living organism is classically conditioned to certain actions. Such as ccebr said rivals/mating calls, are all classically conditioned response to danger, or times of heightened arousal. If you do attempt to classically condition your frog to croak/ribbit everytime he is fed...or before you feed him, it will take a LONG time, because conditioning does not happen overnight...Good Luck, and keep us updated.