A 488 (Not Acceptable Here) response was received from the network and the operation failed

I use ucma send a message like this:

Target To = new Target();
To.SipUri = MsgInfo.To;
SignalingSession Session = new SignalingSession(m_endpoint, new RealTimeAddress(To.SipUri));
Session.LocalParticipant.ApplicationContext = To;
Session.OfferAnswerNegotiation = new MyOfferAndAnswer();
Session.Participate();
Session.SendMessage(MessageType.Message, new ContentType("text/plain"), Encoding.UTF8.GetBytes(MsgInfo.Msg));
Session.Terminate();

Session.Participate(); throw a exception:A 488 (Not Acceptable Here) response was received from the network and the operation failed. See the exception details for more information.

TKS.