Private and Public IP Registration Phone Configuration


I am trying to determine if there is a way to get a Snom 370 (and this issue probably affects all voip phones) to register to 2 different asterisk’s servers (or more) if one or more fo the servers is on a private network while the other (or multiple others) is on a public network.

As an example of the complete situation…

Snom 370 has a private address behind a cisco firewall and it can successfully register to an asterisk PBX behind another another cisco firewall with a different private IP/subnet…

Snom 370 ( <-> Firewall <-> Internet w/ VPN Tunnel <-> Firewall <-> Asterisk Server (

Same Snom 370 can register and use a different Asterisk PBX that is on the public internet…

Snom 370 ( <-> Firewall <-> Internet w/ VPN Tunnel (still up but unused for this) <-> Asterisk Server (4.X.X.25)

However, I cannot get both to register at the same time. Whichever one registers first is successful, the other fails with a network failure issue.

I believe the problem is a NAT issue somehow, and I have tried configuring static nat settings in what feels like a 100 different ways without success. Has anyone successfully done something like this can possibly provide me with the correct “formula”?

I should also note that I can successfully register to multiple public Asterisk/SIP peer servers without issue as long as the private tunnel connection is not used/registered.

Thanks for any assistance.