Fortunately, there's an easy solution: Hold down the Alt button while tapping the appropriate letters. The BlackBerry will automatically convert them to the proper numeric tones. If your model has a keyboard where the letters are doubled up, just press twice (while holding down Alt) to get the "second" letter for any given key.
Hey, and while we're on the subject of BlackBerry tips, here's a favorite: At the end of sentence, press the space bar twice to get a period, a space, and automatic capitalization of the next letter. Got one of your own to share? You know where: the Comments!