This document provides a step-by-step guide for configuring a KX-NS PBX to use SIP trunking between two PBXs without using a SIP provider. It outlines 26 steps to program the PBX for SIP registration, assign DDI/CLIP numbers, and configure the dialing plan. Once set up, calls can be placed between extensions on either PBX by dialing the other extension's DDI number and the caller ID will display the calling extension's CLIP number. The guide concludes by confirming the setup can be tested with calls between extensions.