Hi,
Let us assume that the ip address of your SIP provider is 200.200.200.100.
Add a permanent static route to the file /etc/sysconfig/network-scripts/route-eth0 like this:
200.200.200.100/24 via 173.168.0.1 dev eth0
After adding this file, you will need to restart your network service like that:
service network restart
To check that everything was added to the routing table, you can check it like that:
route -n
Do not forget to add all of your local networks to the sip settings module (LAN and the SIP provider's LAN).
Just one warning though:
If you are not sure how to do it, I suggest to work in a console environment to avoid locking yourself out of the server.
Thank you,
Daniel Friedman
Trixton LTD.