Write some Software

I currently have a VoIP SIP PBX (asterisk based). For communication with a specific trunk i need the p-asserted-identity header to be rewritten to a specific format. This is not possible within asterisk, so i need to setup a very simple session border controller. Asterisk PBX <> SBC (rewrites header) <> Trunk. I have understanding of SIP, but no experience in setting up a SBC. Who can help me setup an SBC?