Now that we have reviewed the common pitfalls of NAT, we can go over the various types of NAT situations that you may encounter. Basically you will probably be in a situation where either your phone or PBX is behind NAT talking to a SIP endpoint that is not behind NAT (or vice versa). Even worse, you might end up in the dreaded double-NAT situation where both sides of a connection are independently behind their own individual NAT routers at the same time.
A double-NAT scenario looks like the following diagram:
Let's start with a sane, yet challenging situation where you have a phone at your house that ...
