This was a comment from Erick (Software Engineer) in an earlier post. I thought I'd highlight the tip!
During development of our SIP services, we often need to inspect SIP packets being sent on BOTH sides of the router, i.e. packets as they leave the phone and packets as they arrive to our servers. To watch packets as they leave a phone often, it is easiest for me to use my Mac's ability to Network Address Translate (NAT) one network connection behind another.
In order to do this, simply connect your airport connection to a Wi-Fi network then go to: System Preferences > Sharing > Internet Sharing. Once there, share FROM the AirPort TO Ethernet. Now, plug your phone into the ethernet jack on your laptop, and you've now shared your laptop's internet connection with your VoIP phone.
Once at this point, however, a common issue you may face is that now you've "double NAT'd" your phone if your laptop is already behind another NAT (a router). Read the Knowledgebase article on NAT traversal for more info on this.
With Windows, I believe (from here on it's all untested for me) you can bridge two network connections. Effectively, the difference from the Mac here is that there is no NAT anymore; instead, your phone is getting its own IP address on the same LAN as your laptop.