Cisco ATA186-I1 Installation And Configuration Manual page 61

Chapter 4 Protocol-Specific Configurations
About NAT Support with SIP
MediaPort
The MediaPort configuration parameter specifies which base port the
Cisco ATA 186 uses to receive RTP media streams. This parameter is only used
for the purposes of SIP support behind a NAT.
NATIP
The NATIP configuration parameter specifies the WAN IP of the NAT. Specify
the LAN IP in the StaticRoute configuration parameter.
Cisco ATA 186 Installation and Configuration Guide
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