Hello, I'm having a really strange issue on billing related to late negotiation. The call end up hanged with no BYE. There is a dialer sending thousands of calls through and OpenSIPS to a termination based on FreeSwitch. I have a SIP call flow as below. Some times during the day, when the volume is high, the UAC drops some 200Ok and FS send a reinvite in the middle of the Initial transaction. ----INVITE --------------- Proxy --INVITE ---------------> FS <----200 OK -------------- Proxy <---200 OK- ------------- FS <--REINVITE--------------- Proxy <-REINVITE--------------- FS ---481 leg does not exit-> Proxy ---481------------------> FS <-- ACK (REINVITE)-------- Proxy <-ACK(REINVITE)---------- FS ----CANCEL ---------------> Proxy ---CANCEL --------------> FS <---200 Ok ---------------- Proxy <----200 Ok ------------- FS FS is sending a reinvite before ACK comes from the client. I have two questions: 1) Is it valid to send a reinvite in the middle of an existing transaction? According to the RFC3261 Section 14. Note that a UAC MUST NOT initiate a new INVITE transaction within a dialog while another INVITE transaction is in progress in either direction. 1. If there is an ongoing INVITE client transaction, the TU MUST wait until the transaction reaches the completed or terminated state before initiating the new INVITE. 2. If there is an ongoing INVITE server transaction, the TU MUST wait until the transaction reaches the confirmed or terminated state before initiating the new INVITE. But it is confusing because just below it says the opposite. However, a UA MAY initiate a regular transaction while an INVITE transaction is in progress. A UA MAY also initiate an INVITE transaction while a regular transaction is in progress. 2. Shouldn't FS send a BYE after sending a 200Ok and not receiving the ACK? If a UAS generates a 2xx response and never receives an ACK, it SHOULD generate a BYE to terminate the dialog. Best regards, Flavio E. Goncalves