Ideally, move the WAN interface of the TA900e to a public IP in front of the firewall.
Is the issue with the analog interfaces on the TA900e or with a SIP phone behind it?
Does the firewall have the capability of doing SIP NAT fixup?
Does the SIP server have the ability to detect whether SIP clients are behind a NAT?
Unfortunately all our devices will be behind a corporate firewall which we do not control. Our SIP Trunk cannot detect whether clients are behind a NAT.
I found this on the web? Not sure if its what I need.
TA900e(config)# ip firewall
TA900e(config)# ip access-list standard PUBLIC
TA900e(config-std-nacl)# permit any
TA900e(config)# ip policy-class PRIVATE
TA900e(config-policy-class)# nat source list PUBLIC address 18.104.22.168 overload
TA900e(config)# interface eth 0/1
TA900e(config-eth 0/1)# access-policy PRIVATE
TA900e(config-eth 0/1)# exit
TA900e(config)# write mem
You may have a difficult time setting this up without knowledge of the firewall in front of the ADTRAN. As Jay mentioned, it is best practice to have the ADTRAN in front of the firewall. If the firewall is in front of the ADTRAN, it must be SIP aware and able to NAT the layer 3 information as well as the SIP information inside the layer 3 packet. The corp firewall is the first place I would look. I do not think that setting up the NAT on the ADTRAN will help much if the ADTRAN is not on the edge of the network.
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