Add static route

Hello
I'm currently installing a DGW L20X and I have one SIP endpoint connected to eth0 and other sip endpoint connected to eth1.
I need to send VoIP traffic through both ports, eth0 and eth1. In the network configuration, for both LAN and WAN I'm forced to choose which of those is going to be "default route". The one that is default is the one that the traffic is sent through. How can I do to send traffic to both ports?
I tried adding static routes but I get an error message every time I try to add one "Failed to create static route".
Thanks in advance for your help

use route add -net command at ssh

Thanks
So i connect via SSH and run the route add command? do you know exactly the syntax I should use?
Thanks

You can google for how to use the route add command
or contact my skype:dario.hu2 and prepare teamview for remote help.