Your remote router in your room upstairs need to have a "bridge mode". If you can configure it as a bridge, you will be able to span between the downstairs LAN and your upstairs LAN.
http://www.dd-wrt.com/wiki/images/3/3a/Standard_bridge_large.jpg
If you can't bridge your two routers, you will need to get yourself another wireless router that can functions as a bridge, or get a couple of Wireless Access Points to do the bridging for you.
http://linksys.custhelp.com/cgi-bin/linksys.cfg/php/enduser/std_adp.php?p_faqid=4194
If you can get your wireless routers, or access points, to bridge then you should be able to use VoIP on your upstairs PC, or plug a VoIP ATA box into one of the bridged router ports.
You will need to configure both wireless routers used in bridging to Port Forward or Port Trigger the appropriate VoIP SIP and RTP ports required for VoIP to function properly. This is always a primary consideration when using VoIP behind NAT-Routers and Firewalls.
http://blog.cqvoip.net/search/label/Firewall