Be sure you are using a touch-tone phone. If you have trouble dialing the destination number after receiving the call back, please check that you are using a touch-tone phone. To check if your phone is set to pulse or tone, pick up your phone and press any button. If you hear a tone when you are pressing the button, your telephone is set to tone dial. If you hear clicking, it is set to pulse dial.
Many phones have a little switch which can change between tone and pulse.
Switch it to tone and try again. If your exchange requires pulse dialing, you can still use the callback system. To check if your exchange uses pulse, make sure your phone is switched to tone, then pick up your touch-tone phone and listen for a dial tone. After you hear a dial tone, press any button between zero and nine and release it. The dial tone should stop.