On my D-Link DIR-655 I have DHCP set and have selected that none of the IP's be reset.
If I remove the D-Link router from controlling/holding the IP's and trying to see if my VOIP device a Grandsteam VT502 provided to me by my Phonepower service will reassign all my IP's.
I need a work around...
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I googled GrandStream VT502 and got two hits - one of which led me to a GrandStream HT502 (which I suspect is the same thing - with the VT version being supplied by a VOIP provider called ViaTalk.
Interestingly enough - it seems that ViaTalk's tech support would rather have you put their hardware infront of your router and leave you to figure out how to make your stuff work, that tell you how to get their hardware to work properly behind a router.
Anyway - it would appear that the HT502 acts as a basic router - you should be able to connect a network switch to the LAN port on the 502 and connect your PCs and have them access the internet, and, if you put your DNS-323 on that switch and configure the dyndns client on the DNS-323 to update dyndns, and setup port forwarding on the 502 - you might get the ftp server accessible from the internet.
The only thing you'd be lacking would be wireless access.