We have installed a PBX on AWS and connected it to our on-prem Router via VPN.

My on-prem router is connected to the SIP provider via a physical connection with another on-prem MUX device (device given by sip provider).

All connections are working fine, EXCEPT, my SIP provider has a condition that all connections to their server must originate from a specific IP that they have assigned to us.

Since AWS machine is connected via VPN, all calls from PBX are picking up the IP of the AWS machine as "source IP".

For resolving this, i need to replace / masquerade / NAT / change the IPs of all connections from AWS machine's IP to SIP provider's assigned IP. Someone suggested i need NAT loopback/reflection for this. Someone also suggested packet forwarding. someone suggest IP masquerading.

Please guide how can this be done?

network topology

1 Answer 1


That looks like unnecessarily complicated setup. Having your SIP phones connected to the AWS PBX over VPN and then back again to the MUX and then presumably back again over the same internet link to the SIP provider means that some of the traffic goes through your three times.

Can you ask your SIP provider to whitelist your AWS SIP EC2 instance's Elastic IP and set up SSL for communication between the PBX and the SIP provider?

That will make the setup much cleaner.

  • Yes, i agree what you are suggesting is more cleaner and better than current. However, in my country it is not allowed which is why they have assigned a specific IP. Or maybe it's their systemic restriction.
    – vichar
    Oct 3, 2018 at 7:46

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