The manual says NOTHING about this, but from experience with other phones, I am going to suggest that you check the various volume levels:
Alert volume
Touch-tone Volume
Keypad volume
... and whatever else you see in the menu. One of these probably holds the key to getting your phone to do what you want.
Your cell provider knows the phones pretty well, so if it doesn't work when you adjust volume levels for the various features, then I would take it in and have them show you how to do it.
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