Unfortunately this has nothing to do with the UM8500. You have to hit the same numbers that correspond to the letters on a normal phone. I know it stinks, but as I said it is a Blackberry issue. I wish they would make it so that the letter on the QWERTY keyboard sent out the appropriate DTMF tone, but to my knowledge they don't yet.
However, when dialing a number initially you can use letters and it converts them to the appropriate digit to be dialed. It just doesn't work after the call is established.