Can this be done?

We have an MP5000, and use SIP phones in another office with a VPN, but would like to see if I can get them setup for use at my house over the internet with no VPN.

We are using a SoniWall TZ-170 w/ Enhanced OS as our firewall which has a section for VOIP & SIP setup.

I have NAT'd a public IP to our MP5000s internal IP. Also turned on the DMZ for VOIP gatekeeper in the Sonicwall.

I have also allowed DMZ for the EP300 on my home router.

So far no luck, any suggestions?