how to re-establish connection between end users in UCMA?

after remote user closed his Lync chat window, the state of conversation is Terminated. how can i reestablish them?

what is wrong with my doing?( it throws "The operation is invalid in the current object state (Terminated)")

any return will be appreciated.

void conversation_StateChanged(object sender, StateChangedEventArgs<ConversationState> e)        {            if (e.State == ConversationState.Terminated)            {                _terminated = true;            }            if (e.State == ConversationState.Established)            {                _terminated = false;            }        }

if (_terminated)                {                    imCall.BeginEstablish(null, null, (ar) =>                    {                        flow = imCall.Flow;                        _callEstablishComplete.Set();                    }, null);                    _callEstablishComplete.WaitOne();                }