"One of the most common issues that people come up against is that they’ve rolled out one FreePBX machine, and now they want another, somewhere else. And to make it just that little bit harder, they want to link them together. At first, this can be a bit daunting, but it’s not - when you know how."
http://www.freepbx.org/2006/10/26/connecting-two-freepbx-machines-together/