UCWA multi-party conversation

May I question about UCWA multi-party conversation development?

I want to create a Multi-party IM conversation using the UCWA API's in iOS application. I was under an impression that I need to first create a peer to peer Conversation first, and then using the addParticipant API, I need to add another participant(s) to the existing conversation.And then I can conversation with all the participants.

The problem is that when arrived participant invitation from the other people, I accept the invitation, and can participate the convervastion.But when send a POST request on the sendMessage (UCWA) , I got the Http Status Code 409 Conflict Error.

The problem occured whent UCWA application to UCWA applcation. When UCWA application to MS Lync ,can communication without problem.

Any help appreciated.

Any advice about the step of multi-party IM Conversation?

Thanks in advance