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The underlying channel technology for the rtp/udp encapsulation and transport is the existing UnicastRTP (chan_rtp) channel driver so you'll see ChannelVarset events for the . The chan_rtp channel driver sets the UNICASTRTP_LOCAL_ADDRESS and and UNICASTRTP_LOCAL_PORT channel variables which will contain the ip with the local IP address and port Asterisk is using for the channel. You'll use these should you need to that media can be sent to. If you want to use this to inject media into the bridge the External Extecrnal Media channel is a member of you can retrieve these channel variables using the normal ARI mechanism to get their values.