When you are in a call, the keypad button can be pressed to display a keypad to use your touch tones. When a touch tone is pressed though, one doesn't hear anything. This is totally normal, and touch tones will work normally even though you can't hear them.
The reason for this is simple. GSM cell phones do not transmit DTMF tones as many people probably think. What actually happens is when you're on a call and press a number or # or *, a signal is sent to the cellular company's equipment indicating you've pressed that button. The tone is then generated in the cellular company's equipment, not on your cell phone. The tones some people hear on their GSM cell phones when they press a number or # or * don't really have a function other than making the person using the cell phone feel good, letting them know that they've pressed a button.
MacBook Pro Mac OS X (10.4.10)