On the BES devices, I have set to show my Number to 'YES', but when we ring out from the BB, the receiver see unknown number.

any idea what I need to do to enable my BB number be visible by the receiver of the call ?

Are your call recipients capable of processing caller ID information?

Probably an issue with the cell provider if everything is set correctly on the BlackBerry. 99% of the time that is where I see the problem.

The 'Show My Number' option is for having your own number show at the top of the phone app. It doesn't affect caller ID. The 'Restrict My Identity' needs to be set to "off." If set to "Always" you'll get "private" on the other end. Usually "unknown number" on the recipient's end is the recipient's issue. Is the recipient another BB, a cell phone, or a landline?