Routing / Multiple Default Gateway Issue

Here is my network topology.

Trixbox with 2 nics. WAN0 on 192.168.8.96 with 192.168.8.1 as gateway. - has its own ISP
Nic 2 is where all the phones are: 192.168.168.12 with no default gateway. Has a different ISP which goes to internal network.

So, here is my problem. Our Phones and VPN is on the 192.168.168.1 network, but since there is no default gateway set up on 192.168.168.1 for the Trixbox, Phones cannot be seen over our VPN at remote locations.

So, how can I set up a 2nd default gateway or what kind of routing do I need to do so Nic 2 talks to 192.168.168.1?

I have found a few forums where you can do a dual default gateway, but I think if I knew how to route correctly, it would be an easy fix I hope. What I need is to have 192.168.168.12 route to 192.168.168.1 which will then in turn let my VPN users at 192.168.4.1/24 see 192.168.168.12.

BTW, I am using WEBMIN to set up routing. Please provide some help as my remote users are deal in the water. Thank you.