You can do this, but you need a device that has SIP ALG or SIP transformations on it. This way you do not use STUN (Network Topology) to amend the local IP address to the public you use the firewall.
This is how I have to setup my SIP as I have Oak call recording which doesnt work with STUN.
So I have a Gamma and a voiceflex SIP trunk going through a Sonicwall (which can have two internet connections going through it). On the SIP Line programming Transport tab - use network topology is set to None, then I have an IP route for each SIP providers network setup to point to the sonicwall. On the Sonicwall SIP transformations is setup and I can either route each provider over a different internet connection or the same one. Depending on the outbound NAT rule configured will depend on the public IP address that is inserted.
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