What is the number you call to have your number come up as unknown? I think it is star 67 or 69, but I am not sure.

Dial *67 before you dial a number (eg, per-call blocking) or call your local telephone company and have them add Caller-ID block to your line (If you use this method to block Caller-ID permanently, you will have to dial *82 before every call for which you do not want to block Caller-ID).