Take the VoIP device out of the loop and see if that corrects your problem.
If it does, you've found the source of your problem.

Your VoIP device should have it's WAN port connected to the cable/DSL modem,
and your WRT54G's WAN port should be connected to the VoIP device. If you've
connected the VoIP device to one of your WRT54G's LAN ports you've most
likely added a second DHCP server to your network, causing you issues.

Usually cable and DSL providers only give you a single public IP address, so
your VoIP woudl be configured by default to connect directly to the
cable/DSL modem and then act as a NAT router for the rest of your network.

Check the documentation for your VoIP device. You can reconfigure your
network a couple ways...

If the VoIP device is acting as a NAT router, you could put your WRT54G into
bridge mode so that it is just an Access Point. The VoIP device would be
doing all the NAT, DHCP, etc. for your network.

You could connect the WRT54G to your cable modem, and connect the VoIP
device to the WRT54G's LAN ports. Consult the documentation for the VoIP
device to determine what Quality of Service settings your VoIP service
requires, and consult the documenation for the WRT54G to learn how to
configure the WRT54G with those required QoS settings.

What I'd do is:
Connect the WAN port on the VoIP device to the cable modem. Configure the
VoIP device to give out DHCP addresses. Setup a DHCP reservation for the
WRT54G. If the VoIP device has a setting where you can specify a DMZ
destination, set your WRT54G as the DMZ. Any incoming traffic that the VoIP
device isn't going to deal with will now be forwared to your WRT54G.

Put the WRT54G in router (not bridge) mode as and use as normal. Because you
configured the VoIP device to forward all traffic to the WRT54G all port
forwarding can be done on the WRT54G.

Optional: Get another Wireless Router, connect it to the VoIP devices WAN
port. Configure this router to serve up guest access to visitors. This way
visitors can get access to the internet, but have no access to your own
network.

I've got an old Linksys router and two WRT54GL's running Tomato in this "Y"
configuration. The old Linksys router gets the public IP, and the WAN ports
of the WRT54GLs connect to the old Linksys router. I've limited the guest
network WRT54GL, turning off things like SMTP and giving the guest network
bandwidth limits, and even limiting what they can access via OpenDNS (
http://www.opendns.com/). Tomato has a neat feature where it will rewrite
all DNS packets so users on the guest network can't specify their own DNS
servers to get around the OpenDNS filters. It was a fun little project and
much apreciated by the parents of my wife's panio students.

-- 
Andrew S. Zbikowski | http://andy.zibnet.us
IT Outhouse Blog Thing | http://www.itouthouse.com
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