Public safety agencies are using LMR network, which has less service availability, less multimedia services and low data rates during the emergency communication. These less services may impact the life in the critical emergency situation. The commercial cellular network provides higher service availability, more multimedia services and higher data rates. The LMR network mobile node can access the cellular network services if cellular/LMR network are interoperable. For accessing the cellular network services, the LMR network mobile node should handoff to cellular network if there is resource available. Ongoing communication may disrupt by this handoff and radio resources very intensive. We have studied Traditional SIP and Seamless SIP method with the constraints of handoff delay and resource utilization. The study presents Session Schedule Manager SIP, which achieves the optimal handoff and increase the radio resource utilization. This Paper intends optimal handoff and maximizes the resource utilization in the interoperable heterogeneous network which provides efficient communication between Disaster responder and Disaster commander.