Port forwarding query

[#1]

Hi there,
Just purchased a Linksys SPA2102 voip adaptor which has an in built router.  I have connected this to my Dlink modem/router 2640B.  The router is downstairs and has the IP of 192.168.1.1 - this has a long cable going upstairs (did go to my PC with IP 192.168.1.2) which now has the voip adaptor plugged into it and then the PC hangs off the router part of that.
My dlink router sees the voip box as 192.168.1.5, it also has the address of 192.168.0.1 for configuration and network connection - however my PC connected to the voip box network socket gets an an IP of 192.168.0.2
My question is that how the hell do I forward port 12345 (example) used for downloading over to my PC with the setup above?? 
previously I used 192.168.1.2 however in my new world setup 192.168.1.2 is now 192.168.0.2 - can my router handle forwarding to that.
I've tried a few things but had no joy as yet!
Looking forward to any help
Cheers, Wayne

[#2]

Buy a cheap switch to share the link so that the PC is logically connected to the router (not the VOIP device).

[#3]

You can set the SPA3102 into Bridge mode. Page 117 of the ATA_Administration guide ( download from here http://www.linksys.com/servlet/Satellite?c=L_CASupport_C2&childpagename=US%2FLayout&cid=116967159807... )
Set the 'Networking Service' on the Lan Setup to Bridge rather than NAT. Also disable DHCP as on page 118.
In this mode the SPA3102 will not allocate a 192.168.0.x address to your PC , instead it will simply pass all traffic to your 2640B which will allocate a 192.168.1.x address and everything will be as it was before.
Hope that helps