Audible Keypad Tone is a facility that can be set on some handsets, which causes them to play a tone when the keys are pressed, so confirming that they have operated correctly. This was introduced to reduce airtime charges to incorrect numbers, although it also provides a better interface and positive feedback for the user.
The sound played is usually the DTMF tone relevant to that particular number, and for consistency, another non-DTMF tone can be played when control keys are pressed