"From Header" when calling SIP URI

Good afternoon all,

Since no one has responded to my post about my specific issue (setting up bulk911.com), I've continued to try to figure this out on my own, and now have a more generic question.

I am using an extension to forward to a SIP URI since the SIP URI begins with a number only.
How do I set the "From Header" when calling a SIP URI? Sending RPID is only an option for trunks, not extensions. No matter what I set my extension's Caller ID to, and no matter what I set the Caller ID of the extension forwarding to the SIP URI to, the "Caller ID" (I assume this would be the SIP From Header) that is passed is always the extension number. How do I change this?

Thanks for the help...

Rob