Problems with * and default gateway

I’m trying to test Asterisk on my LAN at work, so far with no luck, I’ve probably re-installed 6 or 7 times to date. I can connect with SIP softphones such as XLite both on the LAN and from the outside. I have set up all the port forwarding as suggested on various forums, however, once connected voice traffic is one way (I can hear them) and DTMF doesn’t work.

I figured out today that my default gateway is causing the problem, if I remove the default gateway from the IP configuration on the Asterisk box, everything works as it should. Once removed though I can’t reach the box from outside the network.

I have a simple network with 2 24 port DLink switches connected to a Linksys VPN router.

Address of the box would be Mask Gateway Name server

If I remove the gateway address all is well locally, can’t connect remotely.
As you can see I’m behind a router using NAT, I also have a dynamic IP address, have to use a DNS forwarder. My friend has a similar set up at home, the only thing different would be the switches. Am I missing something on the Linux side or is there something I can configure in Asterisk. Looking at some SIP diag logs, we can see the traffic coming in, but the outbound gets lost. Thanks. Anybody?