practice guide about SIP-NAT

As the diagram above shows ,there are  multiple  Session Agents  in both   Access Realm   and Core Realm .   the Session Agents in core realm are  grouped whose strategy is Hunt.   

Calls will be made from  Access Realm and vice versa    . 

the question is that  , the "From" and "To" header in  " INVITE "  request  received in 1.1.10.10  are different  ,however  the   requirement is that the "uri-host" of  "From" header  in "INVITE " Request should be 1.1.99.1 and  the "uri-host" of  "To" header  in "INVITE " Requestwhen the "INVITE " is being  sent to any Session Agents in the Hunt Group .  I just want to konw  how to configure  the sip-nat  to satisfy such requirement  ?