2 Routers (1 VoIP and 1 Routers)

I added a VoIP (D-Link DVG-1120M) to my Network that uses a (D-Link Di-624)all the PC’s are IP assigned and 1 is my FTP server that was lost when I added in the VoIP box. The Techs from the VoIP service had me setup the port forwarding for the server which of course didn’t work and I’m looking for a little help in figuring this out, Thanks in advance.