I am using pjsua2 with python 2.7, in single-threaded mode: ep_cfg.uaConfig.threadCnt = 0 ep_cfg.uaConfig.mainThreadOnly = True I initiate two outgoing phone calls though my ISP, and when the first call is answered, I attempt to cancel the second, unanswered call, like this: second_call.hangup(pj.CallOpParam(True)) This initially seems to work, the hangup() invocation returns and the call becomes DISCONNECTED, but then my application prints these messages and terminates: 23:24:12.655 sip_transactio ...Unable to register CANCEL transaction (key exists) 23:24:12.663 pjsua_call.c .Failed to send end session message: Object already exists (PJ_EEXISTS) [status=70015] 23:24:12.663 call.cpp pjsua_call_hangup(id, prm.statusCode, param.p_reason, param.p_msg_data) error: Object already exists (PJ_EEXISTS) (status=70015) [../src/pjsua2/call.cpp:501] terminate called after throwing an instance of 'pj::Error' ... and there is nothing I can do to catch the exception and recover. Any advice on how to cancel a call while in early state will be greatly appreciated.