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Listen Port UDP: It is not necessary to change the protocol of the communication port used
by the gateway.
RTP Starting Port UDP: The initial value of port number for transmitting voice data among
Gateway(s). Each line requires 2 ports (RTP/RTCP). It is not necessary to change these.
For example: If the starting port is 9000, then Line 1 is using 9000(RTP) and 9001(RTCP),