I don't believe that there is a setting that will allow you to silence the dialing key pad (unless it is a new capability in OS7). But, as a workaround, you can temporarily switch your profile from Normal to Silent or Vibrate Only and the key pad will make no sound when dialing a call. I hope that will work for you.
You could call us biased, and you wouldn’t be wrong, because at CrackBerry we love BlackBerry and think you should own a BlackBerry Smartphone! We also work as a community support and marketing partner with BlackBerry and BlackBerry Mobile. Our immense passion and knowledge of BlackBerry by our editors and community members make this the best resource for BlackBerry you’ll find anywhere!