irule for sip session lookup

Hi, i need help on an irule to do session lookup for sip session based on call-id.

 

Scenario:

 

i have outgoing sip session goes to softswitch on an external network, the softswitch returns to the VS on the LTM and loadbalance the session to the pool.

 

problem is that i need the softswitch to return to the origin server that initiate the outgoing sip session

 

thanks