ok, after more tests:
The first time i call Unregister Remote Talker, it says true (but no effect) and the second time False. It’s seems to be normal (except it’s not functionning)
I’ve tested with Remove All Remote Talkers… no error… no effect 
To clear the buffer: FOnlineVoiceImpl::ClearVoicePackets() i think - i have to implement it in your plugin
Any idea?