Faxing to HP All-in-one C6150 Printer from remote location

Hi. I have a network-ready all-in-one printer from HP. It is connected directly to a router; a wireless Linksys, WRTP54G (Vonage). I have a static IP address for my modem assigned by Comcast. When the printer is installed, two printers appear in the printers/faxes Windows control panel dialog, one for the printer (HP Photosmart C6100 Series) and another for faxing (HP Photosmart C6100 Series Fax). Both of these work fine; I can print to the printer, or, if I select the fax printer, I get a fax dialog which asks for a phone number and cover sheet info, after which, the device goes 'off-hook', dials out, and transmits the document from my computer to the intended fax machine. This all works fine. I want to do this remotely over the internet.
I can now successfully print from anywhere from my notebook after adding a new printer (via the add new printer wizard) and selecting port I created on the printer, based on my static IP address. I had to put the printer's IP address in the router's DMZ, thus exposing it to the internet in order for the wizard to 'see' it remotely.
I want to be able to fax as well, but this does not work. When I created the remote printer, a remote fax printer was automatically created. Its properties show that it will use the fax driver and it will likewise use the correct TCP-based port. However, the fax dialog never appears when I print to this. The printer queue dialog shows up for a few seconds and then disappears. There is no error message.
I spoke with HP. They say that remotely, bidirectional communications with my laptop can not be established; the printer can't 'see' my notebook across the net, so, therefore, it can not initiate the fax dialog.
Any ideas on how to make this work? The router supports VPN. If this can be made to work, then there would be no need to pay for internet-based fax gateway services like FaxWave.

Thank you in advance.