I will use my WRT54G to illustrate examples of how I set Port Forwarding for my VoIP services.
First, use your web browser on its LAN side to access the router using its default LAN Gateway address: 192.168.1.1
By default, the User Name is blank, or empty. Password by default is "admin". Otherwise, use whatever values you may have changed it to for your personal preference.
One primary difference I note for Port Forwarding (vs. Port Triggering) is that forwarding requires fixed IP addresses on the LAN side to be known. If your device your are forwarding to has a DHCP assigned dynamic address, the forwarding may not be reliable. Thus, it may be necessary to give your LAN devices a fixed IP address assignment for reliable port forwarding.
Another difference with Port Forwarding is that you also have to decide to forward UPD, TCP, or Both. It is considered safest to only use UDP protocol forwarding, unless you know that your services explicitly require TCP protocol. Skype is one service that seems to want to use TCP when possible, so you may want to set Skype's forwarding to use Both.
Skype picks a random port to use. So if you are a Skype user, you will need to navigate to Skypes Tools Options Advanced Connection window to find out what random port it has selected for you. I myself pick my own personally specified port, which Skype allows you to do. Whatever port is selected, you will need to configure your NAT-Router to Forward this port.
As far as I know, Google Talk currently only requires port 5222 UDP for voice calls.
SIP VoIP services like CallCentric and Voip.ms can be triggered with SIP Ports 5060 - 5065 and RTP Ports 10,000 - 20,000. I tend to use RTP ports 16384 - 16482 only because that is the range my Linksys PAP2T and SPA2102 ATA's default to for RTP ports.
Port Forwarding can make all the difference when it comes to getting your VoIP services to work on your home network.