[Users] Caller ID

Hi I'm having a problem with users whose Caller-ID I would like to hide. I'm aware that the FROM header has to carry the value anonymous(a)anonymous.invalid, for a user to be shown as anonymous in the callees display. But I'm not sure how to accomplish this without violating core rules, concerning the FROM headerss tag! My accounting is done using the FROM header aswell, but I can imagine that I could just place the flag prior to altering the FROM header, if that is the way to go about this problem. I have just installed the OpenSER 1.0.0. I was wondering if the right way to achieve my goal, would be via the UAC module? All help appreciated, and thanks for a great community :-) Kind regards Martin Kjeldsen