You don't, we aren't a proxy, we are a B2BUA, so each leg will have its own call-id!
I seem to remember using the "sip_invite_call_id" variable successfully, which is handy, even for a B2BUA. You still have the From/To tags to differentiate the inbound/outbound dialogs, so there isn't, strictly-speaking, a reason for a B2BUA to not allow to pass-thru the call-id I think ...