[VoiceOps] SIP - Preserving original source number in redirected call

I'm faced with a Lawful Intercept scenario for SIP call forwarding which requires the forwarded call to contain the original source and destination numbers. The source number may or may not be changed on the forwarded leg. I see how the original called number can be preserved in Diversion or History-Info, but how do I preserve the original source number? Is there an RFC for this that I missed, or misunderstood? Is it as simple as using PAI for the forwarded A-number and From for the original? I'm concerned the wrong number might appear to the called party.