I noticed the other day that the phone number field will accept letters. You can actually enter Unknown or Private as a telephone number.
That said, I've never tried/tested it.
Interestingly enough, last week I received a call from 'UNKNOWN' on my 8830 (and ignored it). Later I wanted to call back someone else that was in my call log and accidently clicked on the UNKNOWN one...and it displayed the actual phone number.
__________________ No longer a BES Admin, but it was fun while it lasted!