A passthrough call is handled in much the same manner as a regular VoIP call from a voice gateway perspective. There are a few events and minor changes that a Cisco voice gateway undergoes in preparation for a passthrough call.
The terminating gateway always uses tone detection to go into passthrough mode. After that, the terminating Cisco voice gateway can use three ways to signal the switchover to passthrough to the originating gateway. These methods are as follows:
Named Telephony Events (NTE) messages in the Real-Time Protocol (RTP) stream
Named Signaling Events (NSE) messages in the RTP stream
Messages within the configured voice signaling protocol
The first method is the old legacy method implemented only by ...