Networking :: Cisco Phone Won't Register Going Through Slackware Router

I was given a Cisco IP Phone (model 7962) and CyberGuard Firewall/VPN (model SG300) from my company. So that I can work from home. But I've been struggling for the past couple of days trying to get it to work. The phone successfully configures it's ip; but it hangs on "Registering".

The setup is such:Cisco phone plugs into the CyberGuard and gets assigned a private IP address; 192.168.48.34; from the CyberGuard.

CyberGuard plugs into my Slackware (12.1) box and gets assigned a private IP address; 10.0.0.191; from the dhcpd server on my Slackware box.

Slackware box then masquerades that 10.0.0.191 into my static public address on the internet.

Port 1723 (PPTP) is open in the iptables firewall on my Slackware box. I've also tried disabling the firewall (ie opening all ports) on my Slackware box.

On my Slackware box; via iptraf; I can see traffic between my Slackware box, the CyberGuard, and my company's server.

Traffic such as:

Code:

And on the company's server; the following is observed:

Code:

My company just keeps saying "open port 1723 + 47". But as I stated above; I have opened them. And Internet searches have revealed absolutely nothing. At this point; I am completely lost as to what may be wrong.