The method includes following steps: defines two ports for UAS, first stage port and second stage port; first stage port is in charge of receiving initial request of conversation; the second stage port is in charge of receiving subsequent transaction processing message; after UAS modifies the port content in address message included in unified resource id in header field in INVITE message into second stage port, it will be written into header field of Contact of response message, and the response message with second stage port message is sent to UAC so that reception of the subsequent transaction processing of conversation is appointed to the second stage port; thus, when system is overloaded, the thing only to do by server is to close first port to reject new initial message of conversionor to make simple error handling for INVITE message.